tplimpl: Allow text partials in HTML templates

[bep]

Most obvius benefit of this is to include CSS partials with css file suffix into HTML templates.

A valid workaround would be to rename the file mystyles.html, but that doesn't work too good for external editors etc.

The css partial is a method used in some themes before Hugo 0.20, but then it stopped working.

This commit reintroduces that behaviour.

Note that the regular layout lookups for text templates, i.e. "single.json" will be
prefixed with "_text/" on lookup and will only match in the text collection.

Fixes #3273

[bep]

I have put #3273 into a 0.20.2 milestone. I notice some breaking themes that include partials on the form "mystyles.css", which does not work without also allowing fetching text templates into html and the other way around.

I think we should just put this out there before people start to make clever workarounds. This will make the inline CSS story so much better.

I will do some more manual testing and thinking about this, but please comment.

And note: This is relevant for partials only.

[bep]

Just tested this with my site.

So:

<style type="text/css">
         {{ partial "mystyles.css" . }}
</style>
=>
<style type="text/css">
        ZgotmplZ
</style>

<style type="text/css">
          {{ (partial "mystyles.css" .) | safeCSS }}
 </style>

=>

<style type="text/css">
        body {
	    background-color: blue;
        }
</style>

Which I would say is expected and good behaviour and I have added tests for it. I think I will figure a way for the plain text partial variant to return string; then I think this would be "all good".

[bep]

I'm talking to myself here, but now I'm happy with the implementation.

Text partials will return a string, so if used in HTML templates where it would normally be auto-escaped, you will have to use the safeHTML or simliar func to mark it as safe.

[rdwatters]

This is a great addition and easier than the hack-ish approach I was taking to CRP with printf. If nothing else, I think long-time Hugo users are used to leveraging safeJS, safeHTML, and safeCSS as a matter of habit with Golang templates. Thanks for this added feature and convenience @bep.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.