Don't defer input evaluation to REPL backend

[serenity4]

Fixes #373.

The fix implemented in #367 spawned a new task for the evaluation, but this eventually synchronized with the backend too (through a backend Channel), still causing a deadlock (or at least, in certain circumstances).

This was largely inspired from JuliaDebug/Infiltrator.jl#139, which sidesteps backend synchronization entirely.

[oscardssmith]

Can this have a test added to make sure this doesn't regress in the future?

[serenity4]

Execution via the REPL mode is already tested here, but it seems the terminal-based tests haven't been running since 1.6. Our use of TerminalRegressionTests.jl is broken and it doesn't appear trivial to fix, so I would suggest to merge this and tackle the revival of these tests in another PR.

[serenity4]

#308 was an attempt to fix these tests but it seems efforts have been abandoned since.

[oscardssmith]

@KristofferC can you approve CI?

[ReubenJ]

Nice! Thank you for attending to this. Can confirm that this works for me locally 🥳

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 50.82%. Comparing base (5327ede) to head (268b262).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
src/repl.jl	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   51.03%   50.82%   -0.22%     
==========================================
  Files           9        9              
  Lines         821      850      +29     
==========================================
+ Hits          419      432      +13     
- Misses        402      418      +16     

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[KristofferC]

Tests need to be updated for nightly and this is not tested anyway.

[serenity4]

Thanks @KristofferC!