Upgrade to Emscripten 2.0.29 (#477)

* Run `npm ci` automatically after the devcontainer is created.
This reduces the chance that another dev will forget to do this.

* Upgrade to Emscripten 2.0.29
Changes required:
* Defined EM_NODE_JS environment variable to get rid of
a warning that appears if the NODE environment variable
is set, but EM_NODE_JS is not.
* EXTRA_EXPORTED_RUNTIME_METHODS is now EXPORTED_RUNTIME_METHODS
* No longer pass the `-s LINKABLE=1` option to emcc when compiling. (This is a linktime setting and emcc
warns about not using a linktime setting when compiling now).

Author: Taytay