initial support for support jade/pug templates

[KiaraGrouwstra]

This is an attempt to continue the existing PR 2205 in the absence of its creator, ShadowManu. The scope of the original PR has been left intact here, though I've attempted to add some rudimentary documentation/tests as had been requested of the original PR.

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that they're okay with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[KiaraGrouwstra]

Note: comments after my first post until the googlebot post appear to be from my own repo from a few months ago. Feel free to ignore them.
The original PR by @ShadowManu for this could be found here, where he stated he was having trouble incorporating the requested changes as he was on travel. Given he signed the CLA there as well, I hope that addresses the GoogleBot concern that part of the commits here were his.

Edit: I can reproduce the CI failure at run test:deps, though not using its underlying command node scripts/publish/validate_dependencies.js. Hm.

[ShadowManu]

Just to confirm. I do accept my commits to be contributed to Angular CLI ;)

@tycho01 I'll try to get active, just that I work until night and I'm left too exhausted for the weekends. But I'll try to help wrap this up.

[KiaraGrouwstra]

Sure, that works for me :), I suppose I've been sufficiently excited about getting initial support in I wouldn't mind helping out where possible.

[ShadowManu]

We run pug-cli alongside ng start, so it has been low on priority for our team.

[filipesilva]

Closing, please see #1886 (comment) for context.

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