Implement block scopes and control flow statements (if/else, while, and for loops)

[hand-burger]

Summary

This PR adds support for block scopes and control flow statements in the RCC compiler. It introduces nested block statements ( { ... } ), if / else conditionals, while loops, and C-style for loops.

Changes

1. Lexer & AST:

• Added tokenization support for the new keywords: if , else , while , and for .
• Extended the Statement AST enum with Block , If , While , and For variants.

2. Parser:

• Implemented recursive parsing of block statements containing lists of nested statements.
• Added parser support for conditional constructs: if (cond) stmt else stmt .
• Added parser support for loops: while (cond) stmt and for (init; cond; post) stmt .

3. LLVM Code Generation:

• Redesigned the variable mapping system from a flat table to a vector stack representing block-local scopes to correctly support block-level lexical scoping.
• Migrated code generation to a single-exit function design with a return_bb basic block, avoiding duplicate terminator instructions inside branches and loops.
• Implemented basic block routing and branch logic for if / else , while loops, and for loops.

4. Tests & CI:

• Fixed a nested if warning in src/main.rs to satisfy Clippy CI checks.
• Adjusted codegen unit test assertions to match the single-exit block generation style.
• Added end-to-end integration tests verifying block scoping, if / else evaluation, while loop iteration, for loop step execution, and deeply nested loops and conditionals.