Export unit functions to fix DynamicQuantities test failures

The DynamicQuantities tests were failing because essential unit functions
were not exported from the main module after refactoring.

Added exports for:
- get_unit: Extract units from symbolic expressions and variables
- validate: Validate dimensional consistency of equations
- equivalent: Check unit equivalence between different representations
- screen_unit: Validate and process unit objects
- get_literal_unit: Extract literal unit metadata from variables

These functions are core to both DynamicQuantities and Unitful unit
validation and need to be available in the main module for DQ tests
to pass without requiring the Unitful extension.

Fixes DynamicQuantities test failures in CI.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

src/ModelingToolkit.jl

@@ -296,6 +296,7 @@ export map_variables_to_equations
 
 export toexpr, get_variables
 export simplify, substitute
+export get_unit, validate, equivalent, screen_unit, get_literal_unit
 export build_function
 export modelingtoolkitize
 export generate_initializesystem, Initial, isinitial, InitializationProblem