Add spellchecker action

[danielskovli]

Description

We should have a spell checker action that runs on PRs. The spell checker should be configurable in terms of language -- we still need to decide between British and American english -- and it should be able to refer to a list of exceptions. For instance, Altinn, Maskinporten, Fiks, etc. Terms that are correctly spelled, but are not valid English words.

The system should also be able to understand camelCasing/PascalCasing/snake_casing, in a similar manner to Rider's built-in spellchecker. And ideally be able to exclude all our test projects and other parts of the code we may not wish to be so pedantica about.

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