Added support to disable suggestion selection

[mastrix]

Added support to disable mention suggestion, it's useful when you need to display the whole list but disable the ability to select smbdy.

Checklist (remove this list before you submit the PR)

• Are there tests for the new code? -

Additional review hints (remove this list before you submit the PR)
It shouldn't break any functionality as it's just preventing events.

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[codecov-io]

Codecov Report

Merging #440 into master will decrease coverage by 0.22%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
- Coverage   78.87%   78.64%   -0.23%     
==========================================
  Files          31       31              
  Lines         691      693       +2     
  Branches      107      109       +2     
==========================================
  Hits          545      545              
- Misses        145      147       +2     
  Partials        1        1              

Impacted Files	Coverage Δ
src/MentionsInput.js	68.55% <0.00%> (-0.20%)	⬇️
src/SuggestionsOverlay.js	64.70% <0.00%> (-1.30%)	⬇️
src/Suggestion.js	85.18% <100.00%> (ø)

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[kaungmyatlwin]

@jfschwarz Can you merge this PR? It's useful.

[jfschwarz]

Sorry, I'm no longer maintaining this project.

[rimzoni]

do we have any timeline when this feature will be released?

[jack-lewin]

Thanks @mastrix for this PR - I hope you don't mind if I try to get it merged!

--

Hey @Sigrsig and @steffektif 👋 I think you're the current maintainers as you've merged PRs in this project recently.

Would you be open to merging this if I rebase the PR & add some tests?