Add substring function

[temonk]

Enhanced the substring function:

• Added two overloads: with and without length parameter.
• Improved handling of edge cases:
  • 1-based to 0-based index conversion.
  • Clamping of negative indices and lengths.
  • Handling of out-of-bounds indices and lengths.

Updated README.md to document the substring function.

[Copilot]

Pull Request Overview

This PR adds a new substring function to the Simpra expression language with two overloads (with and without length parameter) and comprehensive edge case handling including 1-based to 0-based index conversion, negative value clamping, and bounds checking.

Key changes:

• Added substring function with two overloads supporting 1-based indexing
• Implemented robust edge case handling for negative indices, out-of-bounds access, and length clamping
• Added comprehensive test coverage for various scenarios including Unicode strings and boundary conditions

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/AltaSoft.Simpra/Types/SimpraString.cs	Enhanced substring implementation with improved edge case handling and 1-based indexing
src/AltaSoft.Simpra/InternalLanguageFunctions.cs	Added substring function overloads to internal language functions
tests/AltaSoft.Simpra.tests/SimpraExpressionTests.cs	Added comprehensive test coverage for substring function edge cases
README.md	Added documentation for the new substring function
Directory.Build.props	Version bump from 1.0.15 to 1.0.16

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