refactor: simplify literal parsing and textification

[wackywendell]

Description

This PR moves literal parsing and textification into dedicated modules:

• src/parser/literals.rs
• src/textify/literals.rs

The goal is to make literal handling easier to extend without keeping large literal-specific helpers inside the expression parser/textifier.

Changes

• Extracted literal parsing from src/parser/expressions.rs into src/parser/literals.rs.
• Extracted literal textification from src/textify/expressions.rs into src/textify/literals.rs.
• Kept expression parsing/textification as the caller of the shared literal implementations.
• Changed literal type suffix rendering to construct a Substrait Type and use the existing type textifier, instead of keeping a separate string suffix table.

API Impact

This is intended as an internal structure change. There is no new public extension API in this PR.

Validation

• cargo check
• just fmt
• just check
• cargo test --doc
• just test