This change adds a script that loads information about supported

languages from a tree-sitter-language-pack checkout. This creates a

file, reviewboard/treesitter/_languages.py, which has two mappings,

MIME_TYPE_TO_LANGUAGE and FILE_SUFFIX_TO_LANGUAGES, which are used

for determining the tree sitter language name to use for a given file.
The compiled grammars that are shipped in tree-sitter-language-pack

don't include any of the grammar metadata, so this script will go into a

a checkout and grab the filename suffixes. This also fetches the

"first_line_regex" key, although that is not currently used for

detection.

Ran script to create the _languages.py file.