Clean up engine selection

[linusg]

Lots of issues here:

• Engine enum is duplicated for no reason

• The Zig build system supports passing an enum type to options natively, with much nicer output:

  error: Expected -Dengine to be of type build.Engine.
  error:   access the help menu with 'zig build -h'
  

  and

  Project-Specific Options:
    -Dtarget=[string]            The CPU architecture, OS, and ABI to build for
    [...]
    -Dengine=[enum]              JS engine (v8)
                                   Supported Values:
                                     v8
  

• buildOptions() is fallible for no reason after using the right options API

• Everything after @hasDecl(build_opts, "engine") is dead code, this is always true

• Even if the intention was to check value for its null state (also defaulted here, build_opts.engine orelse "v8") the build system already adds v8 to the build graph due to Options.engine being non-nullable with a fallback value

• If all of this worked correctly and you could specify the engine via a root declaration, it could not return a value in its engine() function due to EngineType being an exhaustive enum hardcoded in the library

Overall that last part is a nice idea but obviously it's not been tested. It can be added back with all the above in mind at a later point. (e.g. explicit -Dengine=custom to not pull in v8 deps, non-exhaustive enum for custom values, etc)

[krichprollsch]

Hello @linusg
TYVM for your PR and your comments 🙏
We wiil take the time to look at it seriously with @francisbouvier next week.

[francisbouvier]

Hi @linusg. Thank you for your PR.
You're right, it's better like that, LGTM.

Do you mind signing your commits? So I can merge this PR. Thanks

[linusg]

Will have to look into how that works but will give it a try this evening, sure :)

[linusg]

I treat all of my SSH keys as temporary by design and I don't plan to create and maintain a PGP key either, so assuming you mean sign as in https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits I'm not willing to do that.

If you mean adding a Signed-off-by line I can do that, even though I personally don't see any value added by those. Please let me know.

[francisbouvier]

I agree that Signed-off-by does not add any value if the commit it not signed by a PGP or SSH key.

OK I will merge it like that.

[linusg]

Thank you!