[compiler] Infer optional dependencies (behind feature flag)

[josephsavona]

Stack from ghstack (oldest at bottom):

• [compiler] Inferred deps must match exact optionality of manual deps #30840
• [compiler] Enable optional dependencies by default #30838
• [compiler] Handle optional where innermost property access is non-optional #30836
• -> [compiler] Infer optional dependencies (behind feature flag) #30819
• [compiler] Allow inferred non-optional paths when manual deps were optional #30816
• [compiler] Add fallthrough to branch terminal #30814
• [compiler] Add DependencyPath optional property #30813
• [compiler] Wrap ReactiveScopeDep path tokens in object #30812
• [compiler] Stop reusing ScopeDep type in AnalyzeFunctions #30811

Adds a new feature flag, @enableOptionalDependencies which when enabled allows PropagateScopeDeps and DeriveMinimalDeps to infer optional dependency paths (a?.b).

In PropagateScopeDeps we look for specific safe patterns of nested optional member expressions:

• <variable> "." / "?." <property>
• or <nested> "." / "?." <property>

When we find this pattern we record a dependency on the overall chain, so for example in a?.b.c?.d.map() we would record a dependency on a?.b.c?.d (because the outer .map() portion doesn't match the above structure). If the structure doesn't match - for example with a?.b?.[foo(bar)]?.z - then we fall back to the existing behavior which treats everything after the initial portion as conditional (for that last example we'd continue to record a as the dep).

The other portion is DeriveMinimalDeps, which now represents optional access/dependency states and updates the merge logic to handle them. The order of precedence is unconditional > optional > conditional.

Note: the flag is off by default, but i tried enabling it for all fixtures and the results were all either unchanged or had correct optional deps inferred. I'll also try running it internally with the flag enabled, but i think we can proceed with review and fix-forward any issues identified from that testing.