refactor: implement signals-core with untranspiled classes

[jviide]

This pull request converts the signals-core package to use ES6 classes instead of ES5-style prototypes. It effectively reverts the changes introduced in #160.

Notes:

• The classes are not transpiled to ES5: The transpilation is disabled by excluding the transform-classes Babel transform when transpiling the core.
• Some class property definitions use TypeScript's declare keyword. This avoids a redundant property assignment getting added to the class's constructor. There is a comment included that describes this reasoning.
• Computed's prototype chain changes from Computed -> Signal to Computed -> Intermediate -> Signal. The Preact binding needed to be modified to take this into account: setting Signal.prototype.constructor to undefined gets masked and Computed.prototype.constructor is not undefined anymore. Which led to this cursed piece of code by Yours Truly.

The sizes of the build artifacts before and after this change are listed below. In general the number of bytes saved falls around 40-60 bytes, depending on the output format.

Before:
       1487 B: signals-core.js.gz
       1352 B: signals-core.js.br
       1513 B: signals-core.mjs.gz
       1378 B: signals-core.mjs.br
       1498 B: signals-core.module.js.gz
       1369 B: signals-core.module.js.br
       1561 B: signals-core.min.js.gz
       1421 B: signals-core.min.js.br

After:
       1429 B: signals-core.js.gz
       1300 B: signals-core.js.br
       1449 B: signals-core.mjs.gz
       1328 B: signals-core.mjs.br
       1438 B: signals-core.module.js.gz
       1316 B: signals-core.module.js.br
       1513 B: signals-core.min.js.gz
       1379 B: signals-core.min.js.br

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Latest commit: 00be0d4

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[rschristian]

While the size reduction is nice, wouldn't this create a bit of a weird discrepancy? If someone's using Preact and (for some godforsaken reason) needs to support ES5, there's now this weird little bit of ES6 in their otherwise ES5 bundles.

I'm just not sure this makes much sense to land on its own.

[marvinhagemeister]

Ah totally forgot about ES5. Yeah that might make it difficult to merge this PR as long as Preact itself ships like that.

forgot about es5

[jviide]

Yep, good points.