Observable decorator on computed symbol keys

[StephenHaney]

Resolves #2159

1. I added a new test in typescript-tests that tries to use computed property keys... see test #2159 - computed property keys. Please validate this test is a supported use case (I think it is). It was failing before this change (see original issue for details). By the way, I verified the test passes if the key isn't a computed symbol to make sure the test itself is true.

2. Changed decorators.ts and observable.ts to apply their logic to symbol keyed properties in addition to string keyed properties.

PR checklist:

• Added unit tests
• Updated changelog
• Updated /docs. For new functionality, at least API.md should be updated
  I don't think this fix needs any doc updates.
• Added typescript typings
  As all changes were made in TS files, no need.
• Verified that there is no significant performance drop (npm run perf)
  Note: I didn't see npm run perf but I tried out npm run test:performance
  Current develop branch: 66.22 real 67.58 user
  This PR: 62.46 real 63.76 user

[StephenHaney]

Tests are now passing, including the new one for computed symbol property keys. If we think this is a good idea, I'll finish up this PR with changelog updates, etc.

[urugator]

Btw there seems to be quite a lot of functions/objects expecting string as a prop key. I suppose changing it to string | symbol could help preventing these bugs (as a hint for programmer)?

[StephenHaney]

I suppose changing it to string | symbol could help preventing these bugs

I started going down this path but due to the TS limitation around using symbols as index keys, we'd end up having to cast it back to string everywhere to avoid TS compile errors. After TS patches that limitation, I think it'd be a good idea to use the PropertyKey type everywhere.

[mweststrate]

Published as 5.15.0