feat: implement text direction support

[Goues]

This PR adds new functionality to allow RTL text in react-mentions. Under normal circumstances, you can achieve this via CSS direction: rtl, however, the problem with that is that you need to know which direction is going to be used up front or you need to detect it from the textarea value on change. However, HTML supports dir="auto" as attribute on any element that enabled auto detection by the browser.

In our use case, we have a textarea where we expect RTL and LTR text to appear at the same time - you first add the text in your RTL language and then you add Engligh below it. This means we need text direction per paragraph of text. We take this approach everywhere but now we would like to allow mentioning, so we are looking into react-mentions for the functionality.

I called the prop unstable, because I don't thinks it complete and it needs to be improved, but I need help with this by someone who knows the library a bit more. One problem will be a selection that is hard to achieve with multiple wrapping divs.

This also solves #469

[vercel]

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[codecov-io]

Codecov Report

Merging #475 (382f895) into master (5af7025) will decrease coverage by 0.21%.
The diff coverage is 72.72%.

❗ Current head 382f895 differs from pull request most recent head 851b918. Consider uploading reports for the commit 851b918 to get more accurate results

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
- Coverage   78.84%   78.63%   -0.22%     
==========================================
  Files          32       32              
  Lines         695      702       +7     
  Branches      108      109       +1     
==========================================
+ Hits          548      552       +4     
- Misses        146      149       +3     
  Partials        1        1              

Impacted Files	Coverage Δ
src/Highlighter.js	93.93% <70.00%> (-4.37%)	⬇️
src/MentionsInput.js	68.82% <100.00%> (ø)

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