Add support for string nodes to StaticLayoutProvider

[mattpap]

This PR allows to use string nodes with StaticLayoutProvider. Previously only integer keys were allowed. In future this could potentially be generalized to any types, but is out of scope for this particular work. As a side effect of this change, I had to figure out a solution to issue #13378, i.e. resolve deserialization issues of dictionaries. This PR partially supersedes PR #13437. I will refocus that PR from attempting to fix the issue to its actual main point, which is migration from plain objects to Map type. This PR also removes some leftovers after removal of hammerjs, which I noticed when dealing with some type issues.

fixes #12651
fixes #13378

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a2a15c8) 92.54% compared to head (a87efa7) 92.55%.
Report is 1 commits behind head on branch-3.4.

Additional details and impacted files

@@              Coverage Diff               @@
##           branch-3.4   #13618      +/-   ##
==============================================
+ Coverage       92.54%   92.55%   +0.01%     
==============================================
  Files             323      323              
  Lines           20493    20509      +16     
==============================================
+ Hits            18965    18983      +18     
+ Misses           1528     1526       -2     

[mattpap]

The original half-measure approach in this PR didn't fully work, so I had to do things properly. This means allowing Map<string, T> | {[key: string}: T} where previously only {[key: string}: T} was allowed. This means no more hacks or implicit type coercions of any kind. Internally all properties that accept this border type use a proxying mechanism, so that all relevant code can assume Map interface and use get() or set() instead of index signatures and dreadful JS in operator. This in effect means that missing keys are finally handled properly.

This approach is different from what I attempted in PR #13437. There I converted plain objects to Map and tried to use asymmetric setters and getters to tie that on the type-level. This didn't work well, due to mutability of objects and that setters and getters with different types don't integrate well with the rest of TypeScript.