[Feature]: Use Neovim 0.12+ built-in JSONC parser

[mrjones2014]

Did you check the docs?

• I have read all the docs

Is your feature request related to a problem? Please describe.

neovim/neovim#37795

Neovim 0.12 added an option for vim.json.decode to understand JSONC (JSON with Comments).

We can use this instead of the vendored JSONC parser.

This should go in a major release version since it drops 0.11 support.

Describe the solution you'd like

1. Update minimum required Neovim version to 0.12
2. Use vim.json.decode(json, { luanil = true, skip_comments = true }) instead of the vendored Lua parser, it will be much faster this way

Describe alternatives you've considered

Not doing it

Additional context

No response

[mrjones2014]

Alternatively, keep 0.11 support for now and use feature detection to choose which parser to use at runtime. Then drop the vendored one after a little while.

[mrjones2014]

Actually it does not support trailing commas so I am going to mark this as blocked until it does.