Add -s STRICT=1 flag emscripten build

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

Taken from Emscripten documentation here https://emscripten.org/docs/tools_reference/settings_reference.html

Emscripten ‘strict’ build mode: Drop supporting any deprecated build options. Set the environment variable EMCC_STRICT=1 or pass -sSTRICT to test that a codebase builds nicely in forward compatible manner. Changes enabled by this:

        The C define EMSCRIPTEN is not defined (__EMSCRIPTEN__ always is, and is the correct thing to use).

        STRICT_JS is enabled.

        IGNORE_MISSING_MAIN is disabled.

        AUTO_JS_LIBRARIES is disabled.

        AUTO_NATIVE_LIBRARIES is disabled.

        DEFAULT_TO_CXX is disabled.

        USE_GLFW is set to 0 rather than 2 by default.

        ALLOW_UNIMPLEMENTED_SYSCALLS is disabled.

        INCOMING_MODULE_JS_API is set to empty by default.

Making sure the Emscripten build is done in such a way as to be forward compatible feels like a good feature to have.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently