Fix indentation in highlight shortcode

[bep]

Fixes #4717

[bep]

This is a rather fast and crude implementation, just wanted to "test the water" a little. Have tested it with @jmooring 's test site and it looks OK.

This:

• Exposes a new InnerDeindented method on the shortcode context
• This just trims all lines with the whitespace found indenting the shortcode start tag. This will not work if you indent differently inside the shortcode, but it's certainly nice to avoid any tab calculation.

@kaushalmodi @jmooring opinions?

[kaushalmodi]

I confirm with fix.

Test Markdown file

+++
title = "highlight shortcode in a list"
draft = false
+++

Hugo Issue # [4717](https://github.com/gohugoio/hugo/issues/4717)

-   List item 1
    {{< highlight emacs-lisp >}}
    (message "I am in list at level-1 indentation")
    {{< /highlight >}}

    -   List item 1.1
        {{< highlight emacs-lisp >}}
        (message "I am in list at level-2 indentation")
        {{< /highlight >}}

        -   List item 1.1.1
            {{< highlight emacs-lisp >}}
            (message "I am in list at level-3 indentation")
            {{< /highlight >}}
    -   List item 2.1
        {{< highlight emacs-lisp >}}
        (message "I am in list back at level-2 indentation")
        {{< /highlight >}}
-   List item 2
    {{< highlight emacs-lisp >}}
    (message "I am in list back at level-1 indentation")
    {{< /highlight >}}

<!--listend-->

{{< highlight emacs-lisp >}}
(message "And now I am at level-0 indentation")
{{< /highlight >}}

Before the fix, using hugo v0.99.1

https://highlight-shortcode-indentation-fix--hugo-mwe.netlify.app/posts/highlight-shortcode-in-a-list/

(This is deployed using Hugo v0.99.1 as of writing this on 2022/05/30. The same link should show the corrected highlight sc rendering if deployed using a newer Hugo version in future.)

After the fix in this PR

[bep]

@kaushalmodi thanks a lot.

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