This PR fixes an issue where arbitrary values such as
`border-[12px_4px]` were incorrectly producing `border-color` instead of
`border-width` values.
To solve it, I extended the `<line-width>` check to make sure that every
part of the value is a valid `<line-width>`.
In order for a `line-width` to be valid, every part should be one of:
1. A keyword: `thin`, `medium`, `thick`
2. A length: `12px`
3. A number: `0`
Fixes: #17221
# Test plan
1. Added test to verify this works
2. All existing tests pass
448949318e