feat: add INJECTED_SCRIPTS hook for MFE script injection

[sagrodat]

Creates a parallel pipeline to the PLUGIN_SLOTS pattern specifically for injecting raw JavaScript classes into env.config.jsx. Prevents injected scripts from clobbering default external scripts or React components by injecting before the PLUGIN_SLOTS mechanism which uses FPF (frontend-plugin-framework) that this slot mechanism does not need.

How to test this PR

To verify the injection pipeline works, you can drop this simple test plugin into your local environment.

from tutormfe.hooks import INJECTED_SCRIPTS

test_content = """
class TestScriptLoader {
  constructor(data) { this.data = data; }
  loadScript() {
    console.log("Test Script injected successfully from Tutor!");
  }
}
"""

# Injecting into the 'authn' MFE for testing
INJECTED_SCRIPTS.add_item(
  ("authn", "test_script", test_content)
)

Install, enable and save config it:

tutor plugins install test_script.py
tutor plugins enable test_script
tutor config save

Check the generated file at env/plugins/mfe/build/mfe/env.config.jsx to see the script appended, or copy to the mounted MFE's directory and run the MFE to see the console in dev tools.

[arbrandes]

Superseded by #290.