[chore/bugfix] Deinterface text.Formatter, allow underscores in hashtags

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request does the following:

• Deinterface the text.Formatter to make the code a bit simpler.
• Allow underscores in hashtags as long as hashtag is not only underscores (in line with Mastodon's hashtag behavior).
• Refactor and comment some of the internal/text goldmark and parsing stuff, to make it more consistent and (i hope) more readable!

One of the side effects of this is that it's no longer possible to create italicized hashtags _#LikeThis_, however they can still be created *#LikeThis*.

closes #2199

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

This stuff has moved into plain.go

[tsmethurst]

Logic in here was mostly taken from goldmark_extension.go. Just tried to make it more consistent.

[tsmethurst]

Stuff in this file was previously in goldmark_extension.go but I moved it here to reflect that this stuff is about parsing and not about rendering.

[tsmethurst]

This prevents playing silly buggers and making a hashtag like #____________.

Unfortunately silly hashtags are still possible, so stuff like #a_______________________ is OK.

We might want to fix this up later or make it a bit more nuanced in terms of what it allows, but for now I figured this was OK.

[tsmethurst]

Moved these from the more central util package and unexported them, since they were only being used in internal/text.

[tsmethurst]

This and the following function is where underscores get excepted. In other words, these are the actual fixes for #2199