REPL completion fix on 1.12

[palday]

Multiple sources of breakage

• Actually show glyphs for latex or emoji shortcodes being suggested in the REPL JuliaLang/julia#54800; fix adapted from Fix latex symbol completion JuliaLang/IJulia.jl#1147
• REPL: generate tab completion hints on a worker thread to not block typing JuliaLang/julia#57192; some related discussion Strange behavior (race condition?) in LineEditREPL since moving hints to worker thread JuliaLang/julia#58189

Seems like #588 also helped with calling the function pointers from a background thread!

[codecov]

Codecov Report

❌ Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.08%. Comparing base (0324da9) to head (3651b77).

Files with missing lines	Patch %	Lines
src/RPrompt.jl	82.22%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
- Coverage   85.34%   85.08%   -0.27%     
==========================================
  Files          26       26              
  Lines        1713     1730      +17     
==========================================
+ Hits         1462     1472      +10     
- Misses        251      258       +7     

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[palday]

Looks like multiple causes. Additional bisection leads to:

0b9525bf81a1727e18d3342a6da7f8505bc447f5 is the first bad commit
commit 0b9525bf81a1727e18d3342a6da7f8505bc447f5
Author: Ian Butterworth <[email protected]>
Date:   Fri Jan 31 12:08:26 2025 -0500

    REPL: generate tab completion hints on a worker thread to not block typing (#57192)

 stdlib/REPL/src/LineEdit.jl | 110 +++++++++++++++++++++++++++++---------------
 stdlib/REPL/src/REPL.jl     |  10 ++--
 2 files changed, 79 insertions(+), 41 deletions(-)

JuliaLang/julia#57192