Add despacing to DTL to enable removal of blank lines and floating bits of text in generated HTML

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

Add two regular expressions to DTL's core loader to enable removing excess whitespace by using delimiters like `{#-` and `-#}`. These will remove excessive blank lines and floating text from generated HTML.

### Problem

HTML generated by DTL is, um, not beautiful or even okay looking: it often has lots and lots of blank lines and bits of text floating in space.

I know not everyone does a view-source, but I do. I also want to add lots of comments to my DTL templates without adding lots of blank lines to the generated HTML.

### Request or proposal

proposal

### Additional Details

For a more detailed description, see [django-despacer](https://pypi.org/project/django-despacer/).

I'm a frugal pragmatic perfectionist (with deadlines).

As a perfectionist, I'm horrified by the HTML generated by DTL: so many empty lines and bits of text floating in space. Being pragmatic, I accept that perfect whitespace control that generates perfectly indented HTML is unrealistic; so I'll settle for merely better looking HTML. Being frugal, I won't repeatedly pay a price for it (on every request) but I'm willing to pay just once (at template load time).

I'm pretty sure this won't break existing templates. I don't believe a `-` immediately after an opening delimiter or before a closing delimiter is currently valid. So, backward compatibility should be preserved.

### Implementation Suggestions

For demonstrating and testing the idea, I have implemented it as a package on PyPI: [django-despacer](https://pypi.org/project/django-despacer/). It works on my machine 😄.

Rather than it remaining a package, I'm hoping it can be included in core so it's awkward setup (requiring configuration of "loaders") is not necessary.

Start by reviewing the django-despacer package linked in the proposal, then inspect Django's DTL core loader and how template delimiters are parsed. The work is done when equivalent `{#-` and `-#}` whitespace control is supported in core without breaking existing templates, with tests covering blank lines and floating text.

Written by the indexing model from the issue text.