feat: preserve multi file mutation structure

[spinualexandru]

Solves #7

Context

Adds multi-file mutation tracking to hyprlang-rs, enabling configurations that use source = path directives to save mutations back to the correct source file.

Previously, when a config included multiple files via source directives, all mutations would be merged into a single output. Now, the library tracks which values originated from which file and saves changes only to the modified files while preserving the original file structure.

New API

• save_all() -> ParseResult<Vec> - Save all modified files, returns list of written files
• serialize_file(&Path) -> ParseResult - Serialize a specific source file
• get_key_source_file(&str) -> Option<&Path> - Get which file defines a key
• get_source_files() -> Vec<&Path> - List all parsed source files
• get_modified_files() -> Vec<&Path> - List files with pending changes