Dropping Webpack in favour of Esbuild

[Janther]

webpack vs esbuild

	webpack	esbuild
Production build	3.04 s	0.20 s	~15× faster
Test build	0.51 s	0.17 s	~3× faster
standalone.js (gzip)	175.8 KB	172.1 KB	−2.1%
standalone.js (uncompressed)	844.8 KB	796.7 KB	−5.7%
Installed packages	437	384	−53
Build config	69 lines	84 lines	+15

For esbuild

• Much faster builds, which every standalone and browser test run (locally and in each CI bundle job) waits on.
• Fewer dependencies: webpack and webpack-cli and their dependency trees go, replaced by esbuild, browserslist and browserslist-to-esbuild.
• A slightly smaller bundle, mostly because webpack wraps Slang's top-level await modules as async modules. Most of the difference disappears once compressed.

For webpack

• Native support for new URL('./x.wasm', import.meta.url). esbuild needs a small plugin of ours to handle Slang's .wasm files.