Add union types

[Ph0enixKM]

Closes #442

[Copilot]

Pull request overview

This PR adds support for union types to the Amber programming language, allowing function parameters and variables to accept multiple types. This enables more flexible type signatures like foo(x: Int | Bool | Text) where a parameter can accept any of the specified types.

Key changes:

• Added Type::Union(Vec<Type>) variant to represent union types
• Implemented union type parsing with left-to-right associativity using the | operator
• Updated type checking logic to handle union type compatibility

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/modules/types.rs	Core implementation: adds Union type variant, implements parsing logic with try_parse_type and try_parse_simple_type functions, updates type checking with proper subset logic for unions, and adds Display formatting
src/tests/validity/union_types_valid.ab	Validity test covering basic union types (Int|Bool|Text), union of arrays ([Int]|[Bool]), and arrays of unions ([Int|Bool])
src/tests/erroring/union_types_mismatch.ab	Error test verifying type mismatch detection when passing incompatible types to union-typed parameters
grammar.ebnf	Updates TYPE grammar rule to support union syntax with | operator
.gitignore	Housekeeping: adds .agent/ directory to ignore list and removes trailing newline

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