Enable --pretty by default

[njg7194]

Summary

This PR enables the --pretty option by default, making mypy show visually nicer error messages with source code snippets and error location markers.

Changes

• Changed default value of self.pretty from False to True in mypy/options.py
• Updated documentation in docs/source/command_line.rst to reflect the new default
• Updated documentation in docs/source/config_file.rst to show default: True

Behavior

Before (default):

foo.py:13: error: List item 0 has incompatible type "str"; expected "int"  [list-item]

After (new default):

foo.py:13: error: List item 0 has incompatible type "str"; expected "int"  [list-item]
    aa = ["x"]
          ^~~

Users can still use --no-pretty to get the previous concise output format.

Closes #19108

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[wyattscarpenter]

The problem is fixing the test errors, which is what makes #19510 so complicated. Maybe you will find an easier way. But it may be quite difficult.