ChemicalComposition: add configurable output-name separator and prefix namespacing for generated Thermochimica variables

[chaibhave]

Motivation

Current Thermochimica naming can cause two practical problems:

• Name collisions when multiple ChemicalComposition blocks use overlapping elements (e.g. both need Cr).
• Hardcoded : separator in output names (mu:Na, gas_ideal:NiCl2, etc.), which may conflict with user conventions and downstream tooling.

Design

1) Prefix for generated variables

• New parameter: prefix
• If non-empty and missing trailing _, _ is appended.
• Prefix is applied to all variables generated by the action (elements and Thermochimica outputs).
• Example: prefix = salt -> salt_Cr, salt_mu:Cr, etc.

2) Configurable output separator

• New parameter: output_name_separator
• MooseEnum values:
  • colon (default, backward-compatible)
  • slash
  • underscore
• Affects parsing, ALL expansion, and generated/coupled names for:
  • output_species
  • output_element_potentials
  • output_vapor_pressures
  • output_element_phases
• Strict mode by selection: if separator is slash or underscore, colon-formatted entries are rejected.

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Impact

This issue tracks improvements to ChemicalComposition variable naming for Thermochimica workflows:

1. Prefix support for generated variables (already implemented)
2. Configurable output-name separator for Thermochimica output variables (already implemented)

Together, these changes let users avoid variable-name collisions and choose output token separators beyond :. Default behavior remains fully backward compatible (output_name_separator=colon).