Provide a script to create a complete emscripten build

[hoodmane]

This is based on #103065 but for Emscripten. Create a script Tools/wasm/emscripten.py that automates building for Emscripten.

Linked PRs

• gh-126187 Add emscripten.py script to automate emscripten build #126190

[hoodmane]

Closed by #126190.