Add ISOMITTED function for optional parameter detection

[Jython1415]

Summary

• Adds ISOMITTED function as a semantic helper for detecting omitted LAMBDA parameters in Google Sheets
• Uses ISBLANK internally since Google Sheets passes omitted arguments (via double-comma syntax like FUNC(a, , c)) as blank values
• Provides familiar syntax for Excel users migrating to Google Sheets custom functions

Context

This addresses issue #61. While Google Sheets doesn't have true optional parameters like Excel (no [bracket] syntax), users can simulate optional arguments by:

1. Calling with double-comma syntax: MYFUNC(arg1, , arg3)
2. Checking for omission with: IF(ISOMITTED(param), default, param)

Note: Unlike Excel's ISOMITTED, this cannot distinguish between an omitted argument and an explicitly passed blank cell reference—both return TRUE. This is a Google Sheets limitation, not a function limitation.

Test plan

• Linter passes (uv run python scripts/lint_formulas.py)
• README generated successfully (uv run python scripts/generate_readme.py)
• All 171 tests pass (uv run pytest tests/ -v)
• CI workflow passes

Closes #61