Preserve scientific literals with positive exponent signs

[Eric3-jp]

Scientific notation with an explicit positive exponent is split into separate tokens: formatting SELECT 1e+3*2 with use_space_around_operators=True produces SELECT 1e + 3 * 2, changing the SQL.

Allow either sign in the exponent part of the existing numeric-literal rule. Parsing tests cover uppercase/lowercase exponent markers, integer/decimal mantissas, and negative values; formatting tests verify that the exponent stays intact while the multiplication operator receives spaces.

• Regression tests added: all 8 new cases failed before the lexer change; 4 existing scientific-number controls passed.
• Ordinary test suite on Python 3.11.16: 506 passed, 2 xfailed, 1 xpassed. The dedicated DoS-prevention suite and the maximum-recursion regression were excluded.
• ruff check sqlparse/ and git diff --check passed.
• Reviewed documentation impact; this corrects existing numeric-literal support without adding an option.

Prepared with OpenAI Codex assistance for implementation, tests, and review.