Just standardizing built-in extensions to use a `internal sealed class
Icons` for all their non-dynamic icons.
Looks like a LOT of changes, but it's icons all the way down.
**WARNING:** This PR will probably blow up all in-flight PRs
at some point in the early days of CmdPal, two of us created seperate
`Exts` and `exts` dirs. Depending on what the casing was on the branch
that you checked one of those out from, it'd get stuck like that on your
PC forever.
Windows didn't care, so we never noticed.
But GitHub does care, and now browsing the source on GitHub is basically
impossible.
Closes#38081
