We should be able to create custom TokenKind that doesn't fall in any category

[Gipson62]

Proposal: Support Custom Token Kinds in lexer_builder!
Problem:

Currently, when defining language-specific constructs like @extern or #include, the resulting tokens are split into multiple TokenKinds such as [At, Keyword("extern")]. However, some users may prefer a single, unique TokenKind for such constructs that isn't treated as an identifier or keyword.
Proposed Solution:

Introduce a way to define custom token kinds directly in the lexer_builder! macro. For example:

lexer_builder!(
  /*
    Rest of the lexer definition...
  */
  Custom {
    Extern,
    Include
  }
)

With this feature, users could define custom TokenKinds like Extern or Include and create their own systems for processing these tokens.
Example:

A user could implement a function that outputs these custom tokens. For instance:

fn macro_start(state: LexerState) -> Option<TokenKind> {
    // Logic to recognize and return a custom token
}

This approach would give developers more flexibility in building lexers tailored to their language's unique requirements