Sprout tests to double-check our destructuring

I wanted to double-check the semantics of when default values are used, so i 
wrote out some fixtures with sprout enabled.

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-at-array-hole.expect.md

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+
+## Input
+
+```javascript
+function Component(props) {
+  // destructure slot index has a hole in the input, should return default
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [, /* hole! */ 3.14] }],
+};
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const [t18] = props.value;
+  const x = t18 === undefined ? 42 : t18;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [, /* hole! */ 3.14] }],
+};
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-at-array-hole.js

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+function Component(props) {
+  // destructure slot index has a hole in the input, should return default
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [, /* hole! */ 3.14] }],
+};

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-at-explicit-null.expect.md

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+
+## Input
+
+```javascript
+function Component(props) {
+  // destructure slot index has an explicit null in the input, should return null (not the default)
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [null] }],
+};
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const [t18] = props.value;
+  const x = t18 === undefined ? 42 : t18;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [null] }],
+};
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-at-explicit-null.js

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+function Component(props) {
+  // destructure slot index has an explicit null in the input, should return null (not the default)
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [null] }],
+};

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-at-explicit-undefined.expect.md

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+
+## Input
+
+```javascript
+function Component(props) {
+  // destructure slot index has an explicit undefined in the input, should return default
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [undefined] }],
+};
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const [t18] = props.value;
+  const x = t18 === undefined ? 42 : t18;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [undefined] }],
+};
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-at-explicit-undefined.js

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+function Component(props) {
+  // destructure slot index has an explicit undefined in the input, should return default
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [undefined] }],
+};

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-past-end-of-array.expect.md

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+
+## Input
+
+```javascript
+function Component(props) {
+  // destructure past end of empty array, should evaluate to default
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [] }],
+};
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const [t18] = props.value;
+  const x = t18 === undefined ? 42 : t18;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [] }],
+};
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/destructuring-default-past-end-of-array.js

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+function Component(props) {
+  // destructure past end of empty array, should evaluate to default
+  const [x = 42] = props.value;
+  return x;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: [] }],
+};