The tree-sitter build closure is pretty lean by default, but the
optional web-ui requires emscripten to compile the web interface
javascript/wasm code.
This is clearly not worth the increase in build closure size, and
since emscripten is broken more often than not, let’s patch it out by
default. If somebody /really/ needs the web-ui, there is a
`webUISupport` flag.
c505e57173