Fix Ctrl-C handling in REPL

[geofft]

Python's readline module is unable to handle signals when it's called not on the main thread, and it's blocking so we can't call it from the same thread as Trio. Get rid of the additional thread and handle input ourselves.

[geofft]

This is extremely rough, probably misbehaves on Windows, and is probably incomplete in some interesting way. I think it also misbehaves with Python 3.14. All I can say is it mostly works on my machine (Ubuntu 24.04, system Python). I'm mostly posting here for thoughts on the general approach of calling readline via ctypes + inspiration if someone else wants to run with this and finish it.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 77.10843% with 19 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 99.75714%. Comparing base (e590d39) to head (0e25830).

Files with missing lines	Patch %	Lines
src/trio/_repl.py	77.10843%	16 Missing and 3 partials :warning:

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3306         +/-   ##
====================================================
- Coverage   100.00000%   99.75714%   -0.24286%     
====================================================
  Files             127         127                 
  Lines           19274       19353         +79     
  Branches         1304        1314         +10     
====================================================
+ Hits            19274       19306         +32     
- Misses              0          41         +41     
- Partials            0           6          +6     

Files with missing lines	Coverage Δ
src/trio/_repl.py	77.86885% <77.10843%> (-22.13116%)	:arrow_down:

... and 1 file with indirect coverage changes

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