Add source fixing capability and fix routines for L046

[alanmcruickshank]

This follows on from the refactor PRs which I've already merged (#3552, #3567).

Quite a big one, but here's the summary:

• Adds much better test coverage for the elements of .fix_string(). All part of the new linted_file_test.py.
• Adds a new SourceFix dataclass and methods on segments to both add them (via .edit()) and also to retrieve them.
• Adds capability in the linter and linted file to handle source fixes.
• Updates L046 to use the new capabilty.
• Adds test cases for fixing in L046.

Big PR, and a few places that could use some review.

[codecov]

Codecov Report

Merging #3578 (faa1c68) into main (c2f530c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3578   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          174       174           
  Lines        13149     13205   +56     
=========================================
+ Hits         13149     13205   +56     

Impacted Files	Coverage Δ
src/sqlfluff/core/parser/segments/__init__.py	100.00% <ø> (ø)
src/sqlfluff/core/linter/linted_file.py	100.00% <100.00%> (ø)
src/sqlfluff/core/linter/linter.py	100.00% <100.00%> (ø)
src/sqlfluff/core/parser/segments/base.py	100.00% <100.00%> (ø)
src/sqlfluff/core/parser/segments/meta.py	100.00% <100.00%> (ø)
src/sqlfluff/core/parser/segments/raw.py	100.00% <100.00%> (ø)
src/sqlfluff/core/rules/base.py	100.00% <100.00%> (ø)
src/sqlfluff/rules/L046.py	100.00% <100.00%> (ø)

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[barrywhart]

I'll try and review tonight. We're on vacation this week.

[barrywhart]

Great work! I've thought about this for a while, but you made it happen. 💪 🎉