feat(common-expressions): add substr and split string functions

[tusharagrawa]

Description

Adds two new string manipulation functions to the Common Expressions Plugin:

• substr: Extracts substrings with support for negative indices and optional length
• split: Splits strings with custom separators and optional limit

Changes

• Implemented substr and split functions following existing patterns
• Added comprehensive test coverage (13 new tests, 52 total passing)
• Updated documentation with TypeScript signatures

Change Type (required)

Indicate the type of change your pull request is:

• patch
• minor
• major
• N/A

Does your PR have any documentation updates?

• Updated docs
• No Update needed
• Unable to update docs

Release Notes

Added substr and split functions to common expressions plugin that mirror their javascript implementation

[codecov]

Codecov Report

Attention: Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.22%. Comparing base (570c426) to head (51b5669).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/common-expressions/core/src/expressions/index.ts	89.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #671      +/-   ##
==========================================
- Coverage   90.24%   90.22%   -0.02%     
==========================================
  Files         333      333              
  Lines       20605    20634      +29     
  Branches     1991     2002      +11     
==========================================
+ Hits        18595    18618      +23     
- Misses       1994     2000       +6     
  Partials       16       16              

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[spentacular]

Potentially support substring as well, and maybe all the common JS standard lib functions

[KetanReddy]

Definitely a good call, lets make that a followup story because we probably want to determine exactly what we should support from the standard library

[KetanReddy]

#672

[KetanReddy]

Force merging because builds are being flaky but all tests passed during different builds.