Add C23 bool, true, and false keywords (fixes #250)

[MaxwellCalkin]

Summary

Fixes #250 — CCC fails to recognize bool, true, and false in C23 mode.

C23 (N3096 §6.4.1) promotes bool, true, and false from <stdbool.h> macros to first-class keywords. This PR adds support for all three:

• bool: Added as an alternate spelling for TokenKind::Bool (alongside _Bool), following the same pattern already used for static_assert / _Static_assert
• true / false: Added as new TokenKind::True and TokenKind::False variants, recognized by from_keyword() and handled in parse_primary_expr() as IntLiteral(1) and IntLiteral(0) respectively

What changed

File	Change
src/frontend/lexer/token.rs	Added True/False to TokenKind enum, Display impl, and from_keyword() match; added "bool" alternate for _Bool
src/frontend/parser/expressions.rs	Handle TokenKind::True → Expr::IntLiteral(1) and TokenKind::False → Expr::IntLiteral(0) in primary expression parsing

Compatibility

• Code using #include <stdbool.h> continues to work unchanged — preprocessor macro expansion occurs before keyword recognition
• The fast-reject filters in from_keyword() already accept the first characters (b, t, f) and lengths (4, 4, 5), so no filter updates are needed
• No exhaustive match breakage — all other match statements on TokenKind in the parser use wildcard catch-alls

Test case (from issue #250)

// Should compile with: ccc -std=c23 test.c
bool is_even(int number) {
    return number % 2 == 0;
}

int main(void) {
    bool x = true;
    bool y = false;
    return x && !y ? 0 : 1;
}

🤖 Generated with Claude Code

AI Disclosure

This PR was authored by Claude Opus 4.6 (Anthropic), an AI agent operated by Maxwell Calkin (@MaxwellCalkin).