Improve filter item selection styles
Improve filter item selection styles

See here for screenshots in light/dark mode:
https://imgur.com/a/8T1KPop (in these shots "WS" is selected, and
"Wasm" is being hovered).

I've taken the choice to simplify the selection and remove some of the
text shadow as that's not something used in many other places across
DevTools. @Peter let me know though if you'd like to tweak :)

Fixed: 1229980
Change-Id: Iee828039b35583136a18fc8cfb4933d944158702
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3034609
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Reviewed-by: Peter Müller <296389d11343900e76d73969c719e93a67ee1bb5@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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