refactor(compatibility): hoist flag/command lists to package vars

[htoyoda18]

What I did

getCompletionCommands() / getBoolFlags() / getStringFlags() in
cmd/compatibility/convert.go rebuilt their slice literals on every
call, and Convert calls them once per parsed argument in its main
loop. This moves the three lists to package-level vars so they are
allocated once instead of repeatedly. No behavior change: call sites
now reference the vars directly with slices.Contains/range instead
of calling the getter functions.

Related issue

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[glours]

This introduces an unconditional cost where none is needed: compatibility is imported
unconditionally in cmd/main.go, so these package-level slices now get allocated at process init for every invocation, but Convert (and these lists) only ever runs when plugin.RunningStandalone() is true. For every normal docker compose <cmd> invocation.
That's pure waste with zero payoff, whereas before, the getters simply weren't called on that path.

Not sure this micro-optimization is useful honestly

[htoyoda18]

Thanks for pointing this out. I overlooked the fact that compatibility is always imported while Convert is only used in standalone mode.

Given that, this change shifts a small allocation cost from the standalone path to every invocation, so I agree that the optimization isn't worthwhile.

I'll close this PR. Thanks for the review!