Auto merge of #14509 - canaltinova:position, r=Manishearth

Implement background-position-x/y

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This is a WIP PR. Just HorizontalPosition / VerticalPosition implementations are complete. I would like to get early feedbacks about this architecture. Here's some architectural topics to consider:

- I created `HorizontalPosition` and `VerticalPosition` structs for this and used them in `Position` as well. We have decided to split `Keyword` enum, but we need them as unified for `PositionComponent` enum. So I didn't split but I can split it if we prefer to change PositionComponent as well.
- If we prefer Keyword enum like this, we can create a SubPosition(or something like this) instead of HorizontalPosition/VerticalPosition enums since only difference is 2 lines in `parse` functions. We can create a `parse_horizontal` and `parse_vertical` instead and a lot of code duplication can be cleared.
- I couldn't find a good way to use HorizontalPosition/VerticalPosition's parse functions in `Position`'s parse function. It is a bit more complicated. I'm open to suggestions :)
- I don't know much about logical keywords so do I need to do something different? I placed some comments where logical keywords are processing.

Any advice about these?

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14458 (github issue number if applicable).

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- [X] There are tests for these changes

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