VFP Runtime: Implemented StrExtract and Difference functions

[Irwin1985]

Hi Team,

In this PR I have implemented two functions in the StringFunctions.prg file:

1. STREXTRACT()

• Full implementation including all nFlags logic:
  • Bit 0: Case-insensitive search.
  • Bit 1: End delimiter not required (returns rest of string).
  • Bit 2: Include delimiters in result.

2. DIFFERENCE()

• Implemented using the underlying XSharp.Core.Functions.SoundEx.
• Calculates the difference score (0-4) by comparing the SoundEx codes, matching native VFP behavior exactly.

Cleanup

• Removed the empty stubs/definitions from ToDo-S.prg and ToDo-D.prg.
• Updated the XML include paths to point to VfpRuntimeDocs.xml

All test have passed and logic was verified agains VFP9 behavior using local tests.

[RobertvanderHulst]

Irwin,
Did you also create Unit tests?
Can you add these to the pull request?

[Irwin1985]

Irwin, Did you also create Unit tests? Can you add these to the pull request?

Hi Robert, I just push it now.