csvsort: support mixed ascending and descending columns

[Rober1208]

csvsort can currently sort every selected column in the same direction, but cannot sort one column ascending and another descending. This adds a ~ prefix for descending column identifiers, so csvsort -c 'state,~total' data.csv sorts by state ascending and total descending within each state. The prefix also works on ranges; --reverse flips every direction.

The comparison keeps the existing type inference, null ordering, case-insensitive sorting, and stable order for ties. On Windows, csvsort now keeps --columns values literal so a leading ~ is not expanded as a home directory path. An exact column name beginning with ~ still selects that column as before.

Refs #931.

Validation:

• The new mixed-direction regression failed on the unchanged base (6d19c625) with ColumnIdentifierError, then passed after the change.
• PYTHONUTF8=1 python -m pytest -q --disable-warnings: 360 passed, 31 subtests passed.
• flake8 csvkit tests, isort csvkit tests --check-only, check-manifest, and git diff --check: passed.

This contribution was implemented and verified by Codex AI on behalf of Rober1208. It has not had an independent human code review.

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