Group single-quoted literals with implicit aliases

[vhinayindia]

Fixes #518.

Single-quoted literals with an implicit alias are left outside the projection list. For example, SELECT 'fixed_text' alias1, field1, field2 FROM tbl aligns the following columns under alias1 rather than under the complete first expression. A literal in the middle can split the projection into two lists.

Include String.Single in the existing implicit-alias grouping pass, retaining its requirement that the next significant token is an Identifier. Bare and adjacent strings remain literal tokens, preserving #111's behavior. Numeric aliases continue to match all Number subtypes.

The tests cover projection positions, nested SELECTs, empty/escaped/multiline strings, quoted aliases, parent links, round trips, and repeated reindent formatting. This is limited to single-quoted literal aliases; it does not extend prefixed or typed literals, comment-separated aliases, or dialect-specific alias grammar.

Validation on Linux:

• Full suite on Python 3.10.21, 3.11.16, 3.12.14, 3.13.5 and 3.14.7: 533 passed, 2 xfailed, 1 xpassed on each. The xfail/xpass counts match untouched upstream.

• Identical focused tests: 16 failed, 12 passed on untouched source; 28 passed with the fix.

• make lint and git diff --check pass.

• The repository grouping benchmark and a bounded string-alias projection benchmark report linear scaling before and after.

• Additional SQLite checks preserve result rows and column names across formatting; these do not establish behavior for every SQL dialect. Windows/macOS and hosted CI have not been run locally.

• ran the tests (pytest)

• all style issues addressed (ruff)

• changes covered by tests

• changelog updated; no public API changes