feat: support top-level xsl:variable

[gronke]

A stylesheet's global variables are the backbone of a real transform, holding document lookups and localisation that every template reads.

• Each top-level xsl:variable gets a slot, indexed by declaration order.
• The main function evaluates the globals into their slots before it applies templates, with the source document as the context, so a global can read from the input tree.
• Two instructions carry the values: SetGlobal stores a computed value, and LoadGlobal reads it. They live on the interpreter state for the whole run, so a rule or named template reads the same values even though it is invoked as its own function without a captured scope.
• A reference resolves to a local, then a closed-over name, then a global, so a local variable of the same name shadows the global.
• The AST parser gained the xsl:variable declaration, which it rejected before.

Tests

test_global_variable reads a global inside a template, test_global_variable_from_source reads the source document, test_global_variable_references_earlier_global covers declaration order, and test_local_variable_shadows_global covers shadowing.

Not covered

A global whose value references a later global reads an empty sequence, because the values fill in declaration order rather than by dependency.

[gronke]

Upon review I've noticed #145 already implements this. Therefore closing.