support template processing customization

[patricklx]

resolves #585

[chriskrycho]

Hi @patricklx – thanks for the work here, but I am closing this in line with my comment on #585:

Glint does not have the maintainer time commitment to support multiple syntaxes. Given Ember RFC 779, we are committed to supporting loose mode templates for .hbs and strict mode templates in .gjs/.gts with <template>, and we will absolutely not support any other modes. I understand that might be disappointing, but anything else would be dishonest about what we can or cannot support!

Additionally, given Ember’s own direction, I would strongly recommend that you deprecate ember-hbs-imports and move to the form the community will be standardizing on.

Going forward, we do not intend to support environments adding custom behavior/processing as we did in the past: we had some minimal support for that capability because it was necessary while the community worked out the path to a shared design for strict mode, but now that we've landed on <template> for Glimmer/Ember as a whole, we do not expect to support any other authoring modes.