rename references to "offset properties" to "inset properties"

[dbaron]

Now that the offset-* properties in writing modes have been renamed to the inset-* properties, it seems like the references to the "offset properties" or "box offset properties" should instead be to the "inset properties" or "box inset properties", to match the names of the properties.

We should also probably have a defined term for these that we can link to.

[fantasai]

Yeah, the definition would go into the mostly-unmaintained css-position, though. :/