Use JavaScript functions for Component templates

Summary:
## Rationale
**Disclaimer**: This is an incremental step towards more maintainable/readable react-native-codegen generators. In the future, we may want to replace these templates/string concat logic with *something better*. But until we decide what that *something better* is, let's at least get rid of all this gross string find/replace.

Benefits of using Function templates over String.prototype.replace.
- **Self-documenting**: Template Functions enumerate/describe their exact data dependencies in their signature. You no longer have to read the template implementation to see what data you need to pass into the template.
- **Improved Readability**: JavaScript syntax highlighting makes it really easy to see where/how the data is inserted into the templates. Also template variables used be prefixed/suffixed with ::, which made things really confusing in C++ code (e.g: wtf is `::_CLASSNAME_::EventEmitter::::_EVENT_NAME_::`?).
- **Simpler Interpolation**: Don't have to worry about .replaceAll vs .replace, or calling these replace functions with regexes or strings.
- **Template Type-safety**: Ensure that the correct data types are passed to the component templates (e.g: flow will complain if you accidentally pass null/undefined when a template expects a string).
- **Template Type-safety**: Ensure that we don't pass in extra data to templates (this diff catches/fixes instances of this error). Ensure that we don't forget to pass in data to the template.
- etc.

After this diff, both our Component and NativeModule generators will be using template functions. This string find/replace exists no more in react-native-codegen.

This is also a very surface-level change. I made no efforts to simplify these templates. Let's take a look at that later, as necessary.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D32021441

fbshipit-source-id: f8f27069bcbf9d66dcafb7d1411da1f938eb6dcd

packages/react-native-codegen/src/generators/components/GeneratePropsJavaInterface.js

@@ -26,7 +26,19 @@ const {
 // File path -> contents
 type FilesOutput = Map<string, string>;
 
-const template = `/**
+const FileTemplate = ({
+  packageName,
+  imports,
+  className,
+  extendClasses,
+  methods,
+}: {
+  packageName: string,
+  imports: string,
+  className: string,
+  extendClasses: string,
+  methods: string,
+}) => `/**
 * ${'C'}opyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
@@ -35,12 +47,12 @@ const template = `/**
 * ${'@'}generated by codegen project: GeneratePropsJavaInterface.js
 */
 
-package ::_PACKAGE_NAME_::;
+package ${packageName};
 
-::_IMPORTS_::
+${imports}
 
-public interface ::_CLASSNAME_::<T extends ::_EXTEND_CLASSES_::> {
-  ::_METHODS_::
+public interface ${className}<T extends ${extendClasses}> {
+  ${methods}
 }
 `;
 
@@ -253,21 +265,17 @@ module.exports = {
           );
           const extendString = getClassExtendString(component);
 
-          const replacedTemplate = template
-            .replace(
-              /::_IMPORTS_::/g,
-              Array.from(imports)
-                .sort()
-                .join('\n'),
-            )
-            .replace(/::_PACKAGE_NAME_::/g, normalizedPackageName)
-            .replace(/::_CLASSNAME_::/g, className)
-            .replace('::_EXTEND_CLASSES_::', extendString)
-            .replace(
-              '::_METHODS_::',
-              [propsString, commandsString].join('\n' + '  ').trimRight(),
-            )
-            .replace('::_COMMAND_HANDLERS_::', commandsString);
+          const replacedTemplate = FileTemplate({
+            imports: Array.from(imports)
+              .sort()
+              .join('\n'),
+            packageName: normalizedPackageName,
+            className,
+            extendClasses: extendString,
+            methods: [propsString, commandsString]
+              .join('\n' + '  ')
+              .trimRight(),
+          });
 
           files.set(`${outputDir}/${className}.java`, replacedTemplate);
         });