native, markused: implement -skip-unused

[Spydr06]

As @spytheman demonstrated in the #native-backend discord channel.
This is another PR to get #17782 eventually working.

🤖 Generated by Copilot at fcf710a

This pull request adds the -skip-unused flag for the native backend, which allows skipping the generation of unused functions and improves the compilation speed and binary size. It modifies the vlib/v/gen/native/gen.v file to implement the flag, the vlib/v/gen/native/tests/native_test.v file to enable the flag for testing, and the vlib/v/markused/markused.v file to mark only the main.main function as required for the native backend.

🤖 Generated by Copilot at fcf710a

• Implement the -skip-unused flag for the native backend, which reduces the executable size by omitting unused functions (link, link, link)
  • Add a new method is_used_by_main to the Gen struct, which checks if a function declaration node is used by the main function (link)
  • Modify the gen_fn_decl method of the Gen struct, which generates the code for a function declaration, to skip the generation of unused or bodyless functions and print a verbose message (link)
  • Modify the mark_used function in the markused.v file, which marks the functions that are used by the main function or are required by the backend, to only mark the main.main function as required for the native backend (link)
• Modify the test_native function in the native_test.v file, which runs the tests for the native backend, to add the -skip-unused flag to the compilation command (link)

[Spydr06]

On a side note, I must say this new copilot feature seems very helpful

[spytheman]

Good work.

[Spydr06]

When enabling the builtin module (see #17782), it still tries to compile some unused builtin functions, which obviously fail:

[Spydr06]

You can merge it, I'll fix that issue another time.