Complete PostgreSQL CREATE TYPE Support

[LucaCappelletti94]

Adds full parsing for all PostgreSQL CREATE TYPE statement variants. Previously only composite and enum types were supported.

What's New

Range Types - Parse range type definitions with 6 configuration options

CREATE TYPE int4range AS RANGE (SUBTYPE = int4, CANONICAL = fn);

Base Types - Parse low-level type definitions with 19 options (INPUT/OUTPUT functions, ALIGNMENT, STORAGE, etc.)

CREATE TYPE complex (INPUT = in_fn, OUTPUT = out_fn, INTERNALLENGTH = 16);

Simple Declarations - Parse type shells without representation

CREATE TYPE typename;

The AST extends UserDefinedTypeRepresentation with three new variants (Range, SqlDefinition, None) and adds five supporting enums (UserDefinedTypeRangeOption, UserDefinedTypeSqlDefinitionOption, Alignment, UserDefinedTypeStorage, UserDefinedTypeInternalLength) with comprehensive rustdoc. The parser refactors parse_create_type() into modular functions (parse_create_type_composite(), parse_create_type_range(), and base type option parsers) that handle all PostgreSQL type configuration syntax. Tests provide comprehensive coverage with explicit AST verification for all variants and edge cases. Added 17 PostgreSQL-specific keywords, necessary to parse the keywords used in the type options (ALIGNMENT, CANONICAL, INTERNALLENGTH, PASSEDBYVALUE, RECEIVE, SEND, STORAGE, SUBTYPE, TYPMOD_IN, etc.).

Closes issue #2092