[3.14] gh-145219: Cache Emscripten build dependencies, add install-emscripten (GH-145664) (#145790)

Modifies the Emscripten build script to allow for caching of dependencies, and
for automated installation of new EMSDK versions.
(cherry picked from commit ebb150e76ab4988fdcd5e8caa36b9014497573a5)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>
Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>