Check rules and test files with Codespell

Introduces Codespell for finding typos both in text and in code symbols and identifiers, such as function/method names, modules, classes, etc.

The tool turned out to be quite effective at detecting rule pattern bugs where methods and functions have been misspelled and therefore won't detect issues correctly. I have corrected the findings in this MR.

Codespell does not separate camel/pascal-cased words by default, but the regex for extracting words can be configured. I came up with an expression that separates these words, and it seems to work pretty well: https://regex101.com/r/0J3rfQ/1

A custom dictionary has also been added, which currently contains words from the GitLab style guide and some other words that should be replaced with a better alternative.

/cc @gitlab-org/secure/vulnerability-research