[WIP] Implementing TC39 signal polyfill with tansu

[divdavem]

Note that this implementation differs a bit from the current TC39 specifications to take into account the issues I opened:

• Signal.subtle.watched/unwatched callbacks are not only called when a signal is watched/unwatched but also when a signal is read (cf this issue)
• There is a stabilization process happening when calling set in computed (cf this issue)
• Signals are not recomputed when dependent signals come back to the previous values used for last computation (cf this issue)

In this PR I have imported the tests from the official polyfill but I have disabled/updated some of them because of the above changes. I have also added some other tests.

I don't like the changes I have added in the last commit. I think calling watch on a watcher should also update the watched signals, as suggested here and implemented in #146.

[codecov]

Codecov Report

Attention: Patch coverage is 90.43825% with 24 lines in your changes missing coverage. Please review.

Project coverage is 97.72%. Comparing base (b5aebe9) to head (5a4825b).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/wrapper.ts	92.30%	8 Missing and 4 partials ⚠️
src/internal/storeDerived.ts	30.76%	9 Missing ⚠️
src/internal/storeConst.ts	33.33%	2 Missing ⚠️
src/internal/storeTrackingUsage.ts	94.73%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #145      +/-   ##
===========================================
- Coverage   100.00%   97.72%   -2.28%     
===========================================
  Files           16       18       +2     
  Lines          856     1098     +242     
  Branches       133      183      +50     
===========================================
+ Hits           856     1073     +217     
- Misses           0       21      +21     
- Partials         0        4       +4     

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