Add prek.toml for pre-commit hook support

[Repiteo]

Implements support for prek, a tool for handling pre-commit hooks. This will open the door for us to start uniformly applying formatting changes across the repo, and ensuring they're enforced. This will be guaranteed moving forward via GitHub Actions, where prek will always be ran prior to running the various test suites

As of now, this only did a pass with some builtin checks, mainly aimed at fixing inconsistent whitespace/newlines. Excludes areas which are thirdparty/generated, or are otherwise unsuitable for formatting changes. Most notably excludes the entirety of doc/, which could eventually have portions of it added, but it's by far the area of the repo I'm least familiar with and I cannot intuit which areas are "static" versus what should be formatted (could be revisited in a future PR)

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[mwichmann]

unit tests are likely to cause some interesting problems... a number of them are more than a thousand lines long with large chunks not using what we'd consider our coding standard. If you fix a problem in there and the pre-commit wants to format that, but it's in the midst of other lines that would not line up if the changed chunk is formatted, what do you do? Are the tools smart enough for that case?

[bdbaddog]

We're not ready to merge anything like this yet. We'll do it just after a release.
But like @mwichmann we've tried some of this before and found it often breaks some of the tests because white space such checkers remove is important to some tests.

[mwichmann]

we need a separate solution for "trailing whitespace in test text blocks" anyway, it's really irritating to be niggled by that. I wouldn't let that specifically block this one, fwiw.