Rename macro variable AST nodes

[fabpot]

The current name are just wrong as these classes are only used in the context of macros. They were confusing.

[upsun-dispatch]

📋 PR Summary

This PR renames the macro-related AST node classes TemplateVariable → MacroVariable and AssignTemplateVariable → AssignMacroVariable, since the old names were misleading (they are only used in macro contexts). The new classes hold the real implementation while the old names become thin @deprecated subclasses that emit trigger_deprecation notices, preserving backward compatibility. All internal call sites (parsers, ImportNode, Parser) are updated to the new names, and tests plus CHANGELOG/docs cover the deprecation path.

Changes

Layer / File(s)	Summary
new node classes
src/Node/Expression/Variable/MacroVariable.php	New class holding the implementation moved from TemplateVariable (extends TempNameExpression); implements getName/compile identically.
src/Node/Expression/Variable/AssignMacroVariable.php	New class holding the implementation moved from AssignTemplateVariable; compiles the $macros assignment.
deprecation shims
src/Node/Expression/Variable/TemplateVariable.php	Reduced to a deprecated subclass of MacroVariable that triggers a deprecation in its constructor.
src/Node/Expression/Variable/AssignTemplateVariable.php	Reduced to a deprecated final subclass of AssignMacroVariable that triggers a deprecation in its constructor.
call-site updates
src/ExpressionParser/Infix/DotExpressionParser.php	Uses MacroVariable instead of TemplateVariable when building MacroReferenceExpression.
src/Node/Expression/MacroReferenceExpression.php	Constructor now type-hints MacroVariable; updated a code comment.
src/Node/ImportNode.php	Accepts/creates AssignMacroVariable + MacroVariable; legacy branch now wraps the name in a MacroVariable.
src/Parser.php	addImportedSymbol and getImportedSymbol now reference AssignMacroVariable/MacroVariable.
src/TokenParser/FromTokenParser.php	Builds AssignMacroVariable/MacroVariable for imported symbols.
src/TokenParser/ImportTokenParser.php	Builds AssignMacroVariable/MacroVariable for the import target.
docs
CHANGELOG	Adds a deprecation entry for TemplateVariable/AssignTemplateVariable.
doc/deprecated.rst	Documents the class deprecations as of Twig 3.29.
tests
tests/Node/Expression/MacroReferenceTest.php	Switches to MacroVariable and adds a legacy test asserting the deprecation when TemplateVariable is passed.
tests/Node/ImportTest.php	Switches to the new classes and adds a legacy test asserting both deprecation messages in order.
tests/Node/ModuleTest.php	Updates fixture construction to the new classes.