Add Enum Support to Grammar

[pmqtt]

Add Enum Support to Grammar

Description: Extend the existing grammar to include enums, modeled after Rust's syntax. The enums should support:

1. Variants without associated values (e.g., A).
2. Variants with associated values (e.g., B(i64, string)).

Tasks:

• Update the translation_unit in the EBNF grammar to include enum_definition.
• Add rules for enum_variant and enum_variant_types.
  *Integrate enums as valid types for variable declarations.

Acceptance Criteria:

Grammar successfully parses enums like:

enum Result {
  Ok(i64),
  Err(string),
}