Semantic token for keyword 'undefined'

[nlebedenco]

Currently the keyword "undefined" is not classified. It could be tagged as a .keywordLiteral token similarly to how "unreachable" is handled in fn handleToken(self: *Builder, tok: Ast.TokenIndex) !void.

[nektro]

undefined is not a keyword https://ziglang.org/documentation/master/#Keyword-Reference

[haze]

the vim plugin marks undefined as a constant https://github.com/ziglang/zig.vim/blob/3538ae95b469bab6dc15d93067798c87f8968010/syntax/zig.vim#L48