fix: compile with exactOptionalPropertyTypes and noImplicitOverride

[bwbuchanan]

TypeScript consumers of this addon that build with the TS options exactOptionalPropertyTypes or noImplicitOverride currently fail to compile.

This PR fixes the issue by explicitly marking overridden functions as override and adding an explicit | undefined for optional interface & class members that can be assigned as undefined.

[bwbuchanan]

Perhaps this addon's tsconfig.json should also specify those options, to avoid future regressions?

In general, I think that addons should be built with the strictest possible constraints on their types, to avoid breaking downstream consumers that want to use stricter compilerOptions.

[cibernox]

I also think that would be ideal. Compile this as strictly as possible

[bwbuchanan]

@cibernox I just pushed changes to make that happen.

[bwbuchanan]

Hey @cibernox, can this get merged? Would be nice if I could switch back to mainline instead of having to use my private fork to keep glint happy.

[cibernox]

Merged and published!