Remove CSV_HANDLER and FasterCSV naming

[eitoball]

Summary

Remove leftover FasterCSV-era naming now that the gem depends on Ruby's stdlib csv gem exclusively.

Problem

The codebase still carries names from the FasterCSV → CSV migration:

1. CSV_HANDLER constant in lib/comma.rb:

   CSV_HANDLER = CSV

   Used in lib/comma/generator.rb and the Rails renderer option slicing. The indirection no longer serves a purpose.

2. Spec descriptions in spec/comma/comma_spec.rb still say "FasterCSV options" (lines ~64, ~71) even though the implementation uses CSV.

This confuses new contributors who search for FasterCSV or wonder if a compatibility layer still exists.

Proposed approach

• Replace CSV_HANDLER with CSV directly in:
  • lib/comma.rb
  • lib/comma/generator.rb
• Rename spec describe / it strings from "FasterCSV" to "CSV" (behavior unchanged)
• Grep for any remaining FasterCSV references in comments or docs

Files likely involved

• lib/comma.rb
• lib/comma/generator.rb
• spec/comma/comma_spec.rb
• README.md (if any stale references)

Acceptance criteria

• No CSV_HANDLER constant in the codebase
• No "FasterCSV" in spec descriptions or lib comments (unless historical changelog context)
• All CSV generation specs pass unchanged
• No behavior change

Labels (suggested)

refactor, good first issue

Depends on

None. Can be done independently or bundled with #3/#4.