Add anything and until DSL terms for reasoning models

[rs-03]

Summary

Adds the anything and until DSL constructs requested in #1480, so structured generation can let a model produce free-form text up to a delimiter before switching to a structured schema. This is the common reasoning-model pattern:

from outlines.types import anything, either

outline = "<think>" + anything.until("</think>") + "</think>" + either("yes", "no")

anything corresponds to .*. anything.until(delimiter) matches any text up to, but not including, a literal delimiter. Since outlines-core does not support lookaheads, the negative-lookahead form ((?!</think>).)* is expanded into the equivalent alternation, as suggested in the issue:

([^<]|<[^/]|</[^t]|</t[^h]|</th[^i]|</thi[^n]|</thin[^k]|</think[^>])*

Closes #1480

Design choices worth your input

• until excludes the delimiter (it stops just before it), matching the expansion above. The issue notes "we could also decide that until keeps the </think> token"; I went with the excluding version, and I am happy to change it if you prefer keeping the token.
• Only literal string delimiters are supported in this first iteration; until_regex is deferred as suggested in the issue.
• Delimiter characters are passed through re.escape, so a delimiter containing regex metacharacters is matched literally. As a side effect the generated pattern uses, for example, [^/] where the issue illustration wrote [^\/]; the two are equivalent in Python regex, and using re.escape keeps it robust for arbitrary delimiters.

Changes

• Anything term (with an until method) and the anything singleton in outlines/types/dsl.py, plus a to_regex branch mapping anything to .*
• Exported anything from outlines.types
• Five unit tests in tests/types/test_dsl.py covering the expanded pattern, matching behaviour (including newlines and delimiter rejection), special-character escaping, the empty-delimiter guard, and the end-to-end reasoning outline

Ran locally: pytest tests/types/test_dsl.py -k anything (5 passed).