Utilize new contents data from ember-template-imports

[gabrielcsapo]

Summary

in ember-template-imports/ember-template-imports#74 we added the ability to get the contents from the parsed template code so we can utilize that instead of slicing data.

[gabrielcsapo]

@rwjblue i can update the CI version to the right version for 14 so tests can pass, so you want me to do that in this PR or another one and rebase after that is merged?

[rwjblue]

Ya, lets throw a second PR up for bumping the min 14 version

[bmish]

Yeah, a separate PR for dropping old Node versions would be good. We have a big major release planned in #2319. Would like to get this out soon.

[bmish]

Is this a breaking change? If so, let's include it in the upcoming v5 release (#2319). Note that we already dropped Node 12/17 support in the next branch.

[rwjblue]

Is this a breaking change?

Nope! Just some nice API improvements by recent ember-template-imports releases.

[bmish]

Okay, well it depends on Node 12/17 being dropped right? So let's re-target to next and merge it.

[rwjblue]

well it depends on Node 12/17 being dropped right?

Nope! The comments RE: Node 12 were addressed by #2659 (eslint-plugin-import-helpers added an engines config that excluded Node 12, but since CI was generally red anyways we didn't catch it before merging the original PR updating it).