Deprecate duplicate macro definitions

[fabpot]

No description provided.

[upsun-dispatch]

📋 PR Summary

This PR deprecates defining a macro more than once in the same template. Parser::setMacro() now emits a trigger_deprecation notice (targeting Twig 4.0, where it will become a SyntaxError) when a macro name is already registered, and the change is documented in the CHANGELOG, the deprecation guide, and a new legacy integration test.

Changes

Layer / File(s)	Summary
parser
src/Parser.php	Adds a deprecation notice in setMacro() when a macro name is defined more than once, reporting both the first and second definition lines.
docs
CHANGELOG	Adds a changelog entry for the new deprecation.
doc/deprecated.rst	Documents the macro redefinition deprecation under a new Macros section.
tests
tests/Fixtures/macros/duplicate_definition.legacy.test	Adds a legacy integration test asserting the deprecation message and that the last macro definition wins.