feat(gyp-to-cmake): support linker flags

[KeuqS]

Summary

• add ldflags to the parsed GYP target model and validate its shape
• emit target_link_options for target-specific linker flags
• support the same command expansion and escaping behavior as cflags
• add parser and transformer coverage, including command expansions

Fixes #100

Validation

• gyp-to-cmake focused tests: 23 passed
• TypeScript build for cli-utils and gyp-to-cmake: passed
• ESLint on changed files: passed
• Prettier check on changed files: passed
• git diff --check: passed

[kraenhansen]

Retargeted to main and rebased onto it, since this gyp-to-cmake change doesn't depend on next (@KeuqS: I force-pushed your branch, so git reset --hard origin/fix/100-gyp-ldflags before pushing again).