show regex matches inside text input field

[maxheld83]

this has been previously planned in https://github.com/subugoe/biblids/issues/13 and is related to https://github.com/subugoe/biblids/issues/12, but the existing implementation shows the highlights in a text output area next to the input.

Actually doing this inside the input field would make for a nicer UI/UX.

[maxheld83]

this is pretty hard to do actually, b/c there's no shiny widget which allows this, and even on the js side this looks a bit finnicky.
We'd probably need to create a new shiny input binding. This would be both very useful outside of biblids as well as out of scope.

Starting points for future reference:

• https://codersblock.com/blog/highlight-text-inside-a-textarea/
• https://github.com/lonekorean/highlight-within-textarea
• https://codersblock.com/blog/highlight-within-textarea-v2/