feature: update schema source

[tuanha168]

Did you check the docs?

• [X] I have read all the neoconf.nvim docs

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

The schema gist that Neoconf is using is no longer maintained, and some LSPs are already outdated (for example, Volar), and build steps no longer generate JSON files.

Describe the solution you'd like

Perhaps we should consider switching sources, as suggested by the original gist author? Link

Describe alternatives you've considered

Maintaining the old gist seems unlikely, as it would require a significant amount of effort

Additional context

No response