This adds support for searching the terminal buffer using the
`@xterm/addon-search` library. It also adds three options for searching:
regex, case-sensitive, and whole-word. These can be included or excluded
from the search options for `useSearch` depending on whether the search
backend supports it.
I didn't like any of the Font Awesome icons for these toggles so until
we have time to make some of our own icons that better match the Font
Awesome style, I've appropriated VSCode's icons from their [codicons
font](https://github.com/microsoft/vscode-codicons).
To implement the toggle-able buttons for these options, I've introduced
a new HeaderElem component, `ToggleIconButton`. This is styled similarly
to `IconButton`, but when you hover over it, it also shows a highlighted
background and when active, it shows as fully-opaque and with an
accented border.
Also removes the `useDismiss` behavior for the search box to better
match behavior in other apps.
Also fixes the scrollbar observer from my previous PR so it's wider.
The scale of the old icons was inconsistent with the rest of the Font
Awesome kit so I updated the aspect ratio to make it a little narrower.
I also am adding the outline assets to our repo
