Add a lexer for Schemas for Minecraft Add-ons and 4 examples, merging lexers

[MiemieMethod]

Schema is a formal language for the game Minecraft official staff and docs to show data file structures and interface formats, and also for Minecraft Add-ons creators to communicate or teach with each other.

Reference: https://learn.microsoft.com/en-us/minecraft/creator/reference/content/schemasreference/
Community usage example: https://www.mcbe-dev.net/addons/data-driven/manifest.html

This lexer is summarized from official and some community examples for maximum coverage.

Four examples are provided for lexer tests.

Moreover, this commit merged the existing MCFunction and SNBT with the newly added MCSchema language.

[jeanas]

Re CI failure: apparently, you did not make mapfiles, but edited pygments/lexers/_mapping.py directly. In doing so, you forgot the trailing comma in the 1-element tuple ('*.mcschema',). The expression ('*.mcschema') is the same as '*.mcschema', and since a string is iterable, Pygments sees a series of patterns, the first of which is *, hence the strangeness that pygmentize -N test.unknown is recognized as mcschema.

Thus please run make mapfiles and commit the result.

[jeanas]

Please check the CI failure, I think you need to update goldens.

[jeanas]

Thanks!