Add transform callback to process that is overridable (with the default value being what our transform does today)

[NullVoxPopuli]

inspo: #39 (comment)

Blocks:

• Remove template-imports, add content-tag, drop support for the hbs template literals from ember-template-imports (keeping support for ember-cli-htmlbars) ember-template-lint/ember-template-lint#3021
• Use content-tag instead of ember-template-imports typed-ember/glint#655
• Cleanup transforms.js once content-tag has a transform API built in ember-tooling/ember-eslint-parser#56
• maybe others? (css experimentation, etc?)