Refactor parsing utility functions into shared module

[coderabbitai]

The utility functions parse_name_type_pairs and parse_type_after_colon are currently used by multiple AST nodes (Function and Relation). Moving these functions into a shared parsing-utility module would reduce code duplication and improve maintainability.

Background:
These functions were extracted during the implementation of function parsing and are now used by both Function and Relation AST nodes. Creating a shared module for parsing utilities would follow the DRY principle and make the codebase more maintainable.

Reference:

• PR: Implement function parser #40
• Comment: https://github.com/leynos/ddlint/pull/40#issuecomment-[comment_id]
• Requested by: @leynos