fix: DH-18443: Add variableName to ItemContainer

[jnumainville]

Adds a variable name in support of deephaven/deephaven-plugins#1226
Currently the title is passed through the container and that can be overriden to not be the variable name, so I thought this would be a simple solution to bundle that information.

Required for deephaven/deephaven-plugins#1226

[Copilot]

Pull Request Overview

This PR adds support for passing a variable name through the ItemContainer configuration to support better handling of variable names in deephaven-plugins. The change ensures that variable names can be preserved separately from titles, which may be overridden for display purposes.

• Adds optional variableName property to ItemContainer configuration type
• Updates both the class property declaration and constructor assignment to include the new field

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 44.61%. Comparing base (21c06c0) to head (73c72a0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...kages/golden-layout/src/container/ItemContainer.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2525   +/-   ##
=======================================
  Coverage   44.61%   44.61%           
=======================================
  Files         764      764           
  Lines       42800    42800           
  Branches    10766    10766           
=======================================
  Hits        19096    19096           
  Misses      23693    23693           
  Partials       11       11           

Flag	Coverage Δ
unit	44.61% <0.00%> (ø)

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[jnumainville]

Unnecessary, there is a simpler way to do deephaven/deephaven-plugins#1226