vim-patch:9.1.0553: filetype: *.mcmeta files are not recognized

Problem:  filetype: *.mcmeta files are not recognized
Solution: Detect '*.mcmeta' files as json filetype
          (Tomodachi94)

"pack.mcmeta" was added to the JSON tests because that is the most common
filename with that extension.

There are currently 34,000 instances of this file extension on GitHub:
https://github.com/search?q=path%3A*.mcmeta&type=code&p=2

.zip files with this extension have downloads in the millions on sites
like CurseForge:
https://www.curseforge.com/minecraft/search?page=1&pageSize=20&sortBy=relevancy&class=texture-packs

Further reading about the file extension:
https://minecraft.wiki/w/Tutorials/Creating_a_resource_pack#Creating_a_.MCMETA_file

closes: vim/vim#15189

d33a518025

Co-authored-by: Tomodachi94 <tomodachi94@protonmail.com>

runtime/lua/vim/filetype.lua

@@ -591,6 +591,7 @@ local extension = {
   json = 'json',
   jsonp = 'json',
   geojson = 'json',
+  mcmeta = 'json',
   webmanifest = 'json',
   ipynb = 'json',
   ['jupyterlab-settings'] = 'json',

test/old/testdir/test_filetype.vim

@@ -369,7 +369,7 @@ func s:GetFilenameChecks() abort
     \ 'jq': ['file.jq'],
     \ 'jovial': ['file.jov', 'file.j73', 'file.jovial'],
     \ 'jproperties': ['file.properties', 'file.properties_xx', 'file.properties_xx_xx', 'some.properties_xx_xx_file', 'org.eclipse.xyz.prefs'],
-    \ 'json': ['file.json', 'file.jsonp', 'file.json-patch', 'file.geojson', 'file.webmanifest', 'Pipfile.lock', 'file.ipynb', 'file.jupyterlab-settings', '.prettierrc', '.firebaserc', '.stylelintrc', '.lintstagedrc', 'file.slnf', 'file.sublime-project', 'file.sublime-settings', 'file.sublime-workspace', 'file.bd', 'file.bda', 'file.xci', 'flake.lock'],
+    \ 'json': ['file.json', 'file.jsonp', 'file.json-patch', 'file.geojson', 'file.webmanifest', 'Pipfile.lock', 'file.ipynb', 'file.jupyterlab-settings', '.prettierrc', '.firebaserc', '.stylelintrc', '.lintstagedrc', 'file.slnf', 'file.sublime-project', 'file.sublime-settings', 'file.sublime-workspace', 'file.bd', 'file.bda', 'file.xci', 'flake.lock', 'pack.mcmeta'],
     \ 'json5': ['file.json5'],
     \ 'jsonc': ['file.jsonc', '.babelrc', '.eslintrc', '.jsfmtrc', '.jshintrc', '.jscsrc', '.vsconfig', '.hintrc', '.swrc', 'jsconfig.json', 'tsconfig.json', 'tsconfig.test.json', 'tsconfig-test.json', '.luaurc'],
     \ 'jsonl': ['file.jsonl'],