JS function and variable names should be mangled

[CodingMarkus]

Unless exported to other modules, a lot of space can be saved by rewriting variables and function names to shorter versions.

[CodingMarkus]

I'm currently working on that and will offer a pull request once done. I'm almost 90% done with my initial plan but I'm not sure how good the code really is in the end.

I asked an AI to provide samples that can break my code and it had no issues to break it in seconds. I'm currently trying to fix all these issues and hope that once I fixed them, the code will be somewhat decent. Unfortunately I had to redesign my initial idea otherwise some issues were not fixable at all, as my initial plan didn't consider the highly dynamic nature of JS. But with the new test driven approach (I first created a ton of unit tests that all must succeed if the code works correctly) I think the code is now much more robust.

I did not write a full AST parser, which would have allowed for even more powerful optimizations but would also be pretty much overkill for the desired scope and a minifier is not an optimizer to begin with, so it will not alter any syntax, just replace some names. There might still be issues in my scope parser, though.

And there will be an extra command line option, by default the JS minifier will work as before. Mangling names is totally optional. Also I will look into ways to keep the code debugable after mangling the names, as if there is an exception, you will have a named mangled stack, which makes debugging impossible if you cannot map the mangled names back to the original names.

[CodingMarkus]

I'm still not sure about correctness, but the results of mangling the names can be seen below. Also I was able to improve performance by a factor of over 20 compared to my initial implementation (fortunately 99% of all CPU time was only spent in 4 function calls, so I only had to optimize those for a huge speed benefit).

test-js-libs/jquery.js (plain):
   Reduced the size by 50.7% from 287630 to 141922 bytes
test-js-libs/jquery.js (mangled):
   Reduced the size by 63.5% from 287630 to 104974 bytes
test-js-libs/lodash.js (plain):
   Reduced the size by 73.1% from 544098 to 146273 bytes
test-js-libs/lodash.js (mangled):
   Reduced the size by 83.3% from 544098 to 90665 bytes
test-js-libs/moment.js (plain):
   Reduced the size by 44.8% from 173902 to 95933 bytes
test-js-libs/moment.js (mangled):
   Reduced the size by 61.2% from 173902 to 67466 bytes
test-js-libs/react.development.js (plain):
   Reduced the size by 44.5% from 72141 to 40036 bytes
test-js-libs/react.development.js (mangled):
   Reduced the size by 62.1% from 72141 to 27347 bytes
test-js-libs/three.js (plain):
   Reduced the size by 24.5% from 1247235 to 941063 bytes
test-js-libs/three.js (mangled):
   Reduced the size by 38.8% from 1247235 to 763630 bytes
test-js-libs/typescript.js (plain):
   Reduced the size by 30.0% from 8328619 to 5833705 bytes
test-js-libs/typescript.js (mangled):
   Reduced the size by 36.0% from 8328619 to 5334059 bytes
test-js-libs/victory.js (plain):
   Reduced the size by 23.0% from 2135330 to 1644477 bytes
test-js-libs/victory.js (mangled):
   Reduced the size by 52.2% from 2135330 to 1020674 bytes
test-js-libs/vue.js (plain):
   Reduced the size by 43.3% from 342147 to 193993 bytes
test-js-libs/vue.js (mangled):
   Reduced the size by 60.3% from 342147 to 135798 bytes

Currently you need to add --mangle-js-identifiers to get that functionality but having an option that would only apply to js mode could be a bit weird. The option could be shortened to just --mangle or so.

As it may not always produce correct results, still is a bit slow, may not save a lot in all cases, and currently still makes debugging impossible, I would not want to have it applied always by default, so some kind of option is required. The alternative would be to have a different "mode", like there is js and there is `js+mangling" or so.