[3.14] gh-145177: Put node version into emscripten/config.toml. (GH-146156) (#146159)

Configure node version as part of the emscripten build script, and install
that node version if it isn't available.
(cherry picked from commit 91e1312b950e3a98a9e968da02500db127e06f43)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>