Add resource.Templify

[bep]

This relates to #4446 -- but deserves its own issue.

resource.SASS etc. should also work with the output types, and that will enable template support, but it would also be really cool and useful if we could do:

{{ $css :=  resource "main.scss" | resource.Templify .Site |  resource.Sass | resource.Digest | resource.Fingerprint }}

And then in main.scss:

$white:    {{ .Params.colors.white }} !default;
...

This should give some pretty powerful theming options.

Implementation notes:

• text vs HTML
• Maybe custom delimiters ("{{" and "}}")

[hanzei]

Hugo 0.43 added resources.ExecuteAsTemplate. Does this cover the whole issue?

[github-actions]

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