Introduce in-Textarea highlighting

[garretthyder]

This ticket will house the efforts for introducing in-Textarea highlighting for spaces, terms and other items.

Originally the idea stems from this GlotPress issue;
GlotPress/GlotPress#863

This is quite an involved implementation as we'd probably have to retrofit the textarea with a complimentary div which visually replaces the textarea to support highlighting.
We could potentially use a library like this - https://codersblock.com/blog/highlight-text-inside-a-textarea/

Some ideas for what should be highlighted in the text area are;

• highlight ending space
• highlight non-breaking space
• highlight glossary terms with click replace
• highlight … with click replace
• etc

In some cases such as the term highlight and the trailing ... highlight we can take advantage of the 'Fix Warning' functionality being built for 1.5 in #145 so that on-click those highlights can execute replace logic.

[garretthyder]

We may want to look at the Grammarly extension to get some ideas as they have a really nice in-textarea highlight with tooltip that contains replace actions.

[garretthyder]

There's also the 'contenteditable' path as mentioned on GP ticket GlotPress/GlotPress#166

[Mte90]

If we can use a library is better to have less custom code untested a lot but we will find something :-)

[garretthyder]

Agreed a library is going to be best, hopefully something lightweight.
We'll want to avoid WYSIWYG editors and maybe contenteditable now that I look at it more since both support injecting markup (Bold, Italics, etc) which affects the integrity of the string as we shouldn't be injecting markup that's not already present.