Add xgettext wrapper script (#11)

doc/luanti_xgettext.md

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+# `luanti_xgettext`
+
+This script is a simple wrapper around `xgettext` that additionally passes
+Luanti-specific options. In particular, it lets `xgettext` recognize keywords
+that are used for client-side translations in Luanti.
+
+In addition to `core.translate` and `core.translate_n`, this script follows the
+following convention:
+
+* `S(str, ...)` is a wrapper for `core.translate(str, ...)`
+* `PS(str, str_plural, n)` is a wrapper for `core.translate_n(str, str_plural, n, ...)`.
+* `FS` and `FPS` are wrappers that additionally formspec-escape the result of `S` and `PS`, respectively.
+* `NS` and `NFS` are similar to `S` and `FS`, respectively, but they do not translate the string and instead allow the string to be translated later.
+
+Refer to https://www.gnu.org/software/gettext/manual/html_node/xgettext-Invocation.html for further information on the usage of `xgettext`.

luanti_xgettext.sh

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+#!/bin/sh
+xgettext -L lua --from-code=utf-8 \
+	-kS -kNS -kFS -kNFS -kPS:1,2 -kFPS:1,2 \
+	-kcore.translate:1c,2 -kcore.translate_n:1c,2,3 \
+	"$@"