Change handlebars to ember-handlebars so linguist scope tests pass

[evoactivity]

GitHub Linguist will not merge the glimmer syntax because our handlebars scope name clashes with actual handlebars.

github-linguist/linguist#6578 (comment)

I've updated the scope names to ember-handlebars which should make it safe for linguist to merge.

I've quickly tested this change in vscode and all seems to be highlighting correctly but would appreciate someone else testing before we merge this PR.

The next release of linguist is 12 December so getting this merged and released asap would be good.

[lifeart]

Hi @evoactivity! Thank you for PR! Changes seems good.
Quick search on codebase gives me this dependency match:

Are we good that changing things here not break prettier scope?

[evoactivity]

If I search in the prettier repo for that I only find the string in a test snapshot.

https://github.com/search?q=repo%3Aprettier%2Fprettier%20text.html.handlebars&type=code

I believe that's generated at build time and pulled in from https://github.com/ikatyang/linguist-languages

For what it's worth prettier seems to be functioning correctly in my test repo.