Add cflags support to gyp-to-cmake

[huytdps13400]

Fixes #99

Summary

• accept and validate target-level cflags in parsed binding.gyp input
• generate target_compile_options(<target> PRIVATE ...) for those flags
• preserve the transformer's existing command-expansion and space-escaping behavior
• target the resolved CMake name when namespaced targets are enabled
• add a minor changeset for gyp-to-cmake

This intentionally leaves cflags_cc, ldflags, variables, conditions, and target_defaults to their separate tracked issues.

TDD evidence

Parser tests first failed because valid cflags were rejected as an extra property and malformed values were accepted. After adding the parser boundary, transformer tests failed because no target_compile_options command was emitted. Both stages are now green.

Coverage includes:

• valid and malformed cflags input
• non-string array entries
• multiple flags and escaped spaces
• command expansion into a flag list
• namespaced target output

Validation

Node 24.16.0 / pnpm 10.33.0:

• pnpm --filter gyp-to-cmake test — 38/38 passed
• pnpm run build — passed
• pnpm test — 115/115 passed across gyp-to-cmake, host, and cmake-rn
• pnpm lint — passed
• pnpm prettier:check — passed
• pnpm depcheck — passed
• git diff --check — passed