Add CI job for Emscripten

[hoodmane]

The Emscripten buildbot has been around and stable since June of 2025. It also shouldn't be the slowest CI job. We should add an Emscipten CI job.

Linked PRs

• gh-146197: Add Emscripten to CI #146198
• [3.14] gh-146197: Add Emscripten to CI (GH-146198) #146331
• gh-146197: Run -m test.pythoninfo on the Emscripten CI #146332
• [3.14] gh-146197: Run -m test.pythoninfo on the Emscripten CI (#146332) #146336
• gh-146197: Include a bit more information in sys._emscripten_info.runtime #146346