[ENH] Add flag_in_range function (resolves #708)

[naseem173]

Resolves #708
Adds a new flag_in_range function following the same pattern as the existing flag_nulls function. It appends a flag column indicating whether each row's value in a given column falls outside a specified [low, high] range, supports inclusive/exclusive bounds via the inclusive parameter, supports custom output column naming via flag_column_name, and does not mutate the original DataFrame.

Includes tests in tests/functions/test_flag_in_range.py mirroring the existing test_flag_nulls.py test structure, and a CHANGELOG entry.

Note: flag semantics (1 = out of range) were chosen to match flag_nulls's convention. Left a comment on the issue to confirm this interpretation with the maintainer before finalizing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.90%. Comparing base (901f4b3) to head (1f00a61).
⚠️ Report is 192 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1620      +/-   ##
==========================================
- Coverage   87.56%   85.90%   -1.67%     
==========================================
  Files          95      126      +31     
  Lines        6819     9930    +3111     
==========================================
+ Hits         5971     8530    +2559     
- Misses        848     1400     +552     

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