Fix type annotation on var producing duplicate definition error

[rtfeldman]

Summary

• Fixed a bug where a type annotation followed by a var declaration produced a duplicate definition error
• Added anno field to s_var statement to store optional type annotations
• Updated type annotation processing in the canonicalizer to handle .@"var" statements

Test plan

• Added eval test to verify annotated var declarations work correctly
• All existing tests pass
• zig build minici passes

Fixes #8660