Quote source code in messages output

[sim642]

Here's a fun feature that nobody asked for! Quoting of relevant source code a la any normal compiler:

[Success][Assert] Assertion "success" will succeed (tests/regression/00-sanity/01-assert.c:9:3-9:18)
    assert(success);
  
[Error][Assert] Assertion "fail" will fail. (tests/regression/00-sanity/01-assert.c:10:3-10:15)
    assert(fail); // FAIL!
  
[Warning][Assert] Assertion "unknown == 4" is unknown. (tests/regression/00-sanity/01-assert.c:11:3-11:23)
    assert(unknown == 4); // UNKNOWN!
  
[Warning][Deadcode] Function 'main' has dead code:
  on line 13 (tests/regression/00-sanity/01-assert.c:13-13)
      assert(silence); // NOWARN!

Also works for multiline location ranges and in a terminal the range is highlighted with a turquoise color (the only color that our rainbow 🌈 output doesn't yet use).

It is disabled via warn.quote-code by default since getting the quotes for all messages involves opening the files to read out the corresponding lines, which might otherwise negatively impact benchmarking.