Modernize and simplify tooling and code

[mourner]

• Upgraded to Emscripten v5, dropping the Docker-based build in favor of a locally-installed emcc; enabled LTO, and switched to emmalloc (smaller allocator) for a reduced binary size; removed a number of redundant build flags; updated dev dependencies and CI
• Switched to a plain .wasm binary instead of the Emscripten-generated JS+WASM bundle: deleted src/icu.wasm.js, wrote a small manual glue layer in src/index.js that instantiates the WASM directly, and embedded it in the Rollup bundle via @rollup/plugin-wasm
• Reduced memory usage by 50x — previously, WASM build used 16MB memory by default. We can get away with ~256kB since we use the plugin on labels, not huge amounts of text.
• Removed bidi_getLine (the only user of ubidi_writeReordered) and rewrote processBidirectionalText to use the same run-based approach (bidi_setLine → bidi_getVisualRun → bidi_writeReverse) that processStyledBidirectionalText already used; BiDi control characters in LTR runs are now stripped in JS to match the previous behavior
• Cleaned up JS internals: replaced ccall with direct function calls; consolidated malloc + paragraph setup into one helper; replaced per-run heap allocations for output pointers with a single allocation before the loop
• Expanded test coverage: bracket mirroring in RTL context, multiple paragraphs, and pure LTR passthrough

[stepankuzmin]

This is awesome 🔥