Accept (i, name) -> Bool functions for select and drop

[quinnj]

Fixes #1207.

CSV 0.10 accepted a selector function of the form (i, name) -> Bool for select and drop. 1.0 rejected it and pointed users at Tables.Scan. A Scan cannot express a computed rule over header names, such as normalizing each name before matching it against a list, which is the #1207 workflow.

Change

• The function needs only header names, and CSV settles those before it reads any value. _selectpositions now turns the function into the Bool mask it already accepts.
• Every reader that settles columns goes through that one path: CSV.File, CSV.lazy, CSV.File(lazyfile), CSV.Rows, CSV.Chunks, and multi-source vectors.
• As in 0.10, the function gets the column position and the Symbol name (after normalizenames) and must return a Bool.
• types and pool still reject functions.
• Docs: reading guide, examples, migration guide, and the Rows docstring.
• Release: bumps the version to 1.2.0 and adds the 1.2.0 release notes.

Tests

• New select and drop functions testset: File from IO and from bytes, drop, all-false masks, lazy, Rows, Chunks, multi-source vectors, normalizenames, positions under CSV.File(lazyfile), and a non-Bool return.
• The two assertions that the function form throws now check the new behavior instead.
• Local: full suite 331561/331561 on Julia 1.12 (--check-bounds=yes -t4), test/api.jl on Julia 1.10, test/quality.jl 11/11, and the docs build.
• The help migrating to CSV v1.0 #1207 reporter's function works with drop=_drop. It must call String(name) before Unicode.normalize, because the name is a Symbol, as in 0.10.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.75%. Comparing base (a0335ff) to head (4c2e868).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1210   +/-   ##
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  Coverage   95.75%   95.75%           
=======================================
  Files           9        9           
  Lines        5489     5491    +2     
=======================================
+ Hits         5256     5258    +2     
  Misses        233      233           

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