COMPASS-4294: Improve shell plugin interaction

[Anemy]

https://jira.mongodb.org/browse/COMPASS-4294

This PR has 2 small improvements for the shell plugin interaction (changes are in browser-repl):

• Background coloring of cursor selection - I went with gray dark 1 from https://mongodb.github.io/leafygreen-ui/?path=/story/palette--ui open to any suggestions.
• Click to focus input anywhere on a blank input instance (it used to only focus when clicking on the textfield line).

Screenshots

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[alenakhineika]

LGTM. Only some tests need to be fixed before merging. Currently, Windows tests are not passing on evergreen.

[lrlna]

nice ✨

we've been having issues with windows tests. I'd just rerun them again, to make sure it's an actual issue.