feat: Add support for converting regex_syntax::ast::Ast to NFA; Deprecate the old navie AST.

[LinZhihao-723]

Description

This PR deprecates the old naive AST; instead, we use regex_syntax::ast::Ast as our regex AST representation and add support for converting an AST into an NFA.
Notice that not all AST node types are currently supported yet; we support all types needed to build our demo. The current code structure should be very easy to extend for other AST node types.

Validation performed

• Add unit tests to ensure the NFA conversions for supported AST nodes are as expected.
• Ensure DFA's existing unit tests worked with new NFA implementation