Make Fortran, Verilog, and F# parsers opt-in

[thesiti92]

Default CLI builds now omit the Fortran, Verilog, and F# parsers to reduce executable size. Files in those languages continue to produce plain-text diffs with the existing unsupported_language fallback. Structural support can be restored with lang-fortran, lang-verilog, lang-fsharp, or all-languages.

Adds a CLI regression test covering language listing and real sample diffs with optional parsers enabled and disabled.

Validation:

• cargo +stable test --locked -p diffr-cli: 275 passed, 1 existing ignored test.
• cargo +stable test --locked -p diffr-cli --features all-languages: 275 passed, 1 existing ignored test.
• cargo +stable fmt --all -- --check and git diff --check passed.
• macOS ARM64 release builds from the same commit: default 108,894,592 bytes (108.89 MB), all-languages 145,395,744 bytes (145.40 MB). Reduction: 36,501,152 bytes (36.50 MB, 25.10%).
• Both release binaries passed sample diffs for the three optional languages and JavaScript; native parser symbols are absent in the default binary and present with all-languages.

AI assistance: OpenAI Codex.