Names of <input type=range> / <input type=checkbox switch> pseudo-elements

[annevk]

I decided to split this from #4410 as that discussion is already quite long.

In WebKit we accidentally implemented the new pseudo-elements as ::thumb and ::track. They are not shipped and can be renamed, but when discussing it internally none of us really liked the longer prefixed names. Also, when it was revealed we implemented these pseudo-elements, nobody noticed they had the wrong names. @nt1m discovered it while auditing pseudo-elements in general.

Selectors are typically short words, without thematic grouping. E.g., it's :playing, not :media-playing. They get context from usage. E.g., input::thumb.

And for a switch it's also not at all a "slider" thumb. It's just the thumb. It slides while animating, but it's not like you can hold it in any position that's not on or off.

If there's ever a need for another thumb/track-like pseudo-element that can have a longer name.

[yisibl]

RESOLVED: Add ::slider-thumb, ::slider-track
RESOLVED: Define these two pseudos as siblings of each other
RESOLVED: Apply them to <input type=range>, switch, <progress>, and <meter>
RESOLVED: Add ::slider-fill when relevant
RESOLVED: track, fill, thumb are in the same tree order as their usual painting order

#4410 (comment)

I found a problem with the name of the WebKit implementation, but I thought the CSSWG had discussed it elsewhere and removed the slider- prefix from the name.😂

Overall, I like the shorter names.