Add yaml file associations for appmanifest, script_api and ext.manifest

[redoak]

These are not listed in src/data/settings.ts. I've added them locally in my project's settings.json with success.

Suggested addition:

'files.associations': {
    ...
    '*.appmanifest': 'yaml',
    '*.script_api': 'yaml',
    'ext.manifest': 'yaml'

[astrochili]

Looks good!