[Feature] Add VARCHAR(max_length) to type system for user-specified variable-length string limit

[shirly121]

Is your feature request related to a problem? Please describe.
In the current neug type system, variable-length strings are described via string.var_char in schema YAML with optional max_length (e.g. max_length: 65536), but there is no user-facing, explicit VARCHAR(max_length) type. Users cannot use syntax like VARCHAR(256) in DDL or queries to specify the maximum length of variable-length strings, which hinders clear semantics and storage/validation optimizations.

Describe the solution you'd like
Add VARCHAR(max_length) to the neug type system so that users can explicitly specify the maximum length of variable-length strings, for example:

• Support syntax such as VARCHAR(256) and VARCHAR(1024) in DDL or schema definitions;
• Map internally to the existing LogicalType::STRING(max_length) and VarcharExtraInfo;
• Keep semantics consistent with the current YAML schema string.var_char.max_length and handle parsing, validation, and serialization uniformly.

Describe alternatives you've considered

• Rely only on existing YAML var_char.max_length: no DDL/syntax-level VARCHAR(n), so users must hand-write YAML in schema files, which does not align well with unified DDL and tooling.
• Use a fixed default max length (e.g. 65536): users cannot lower the limit when needed, which is less ideal for storage and constraints.

Additional context

• The codebase already has VARCHAR_DEFAULT_LENGTH, VarcharExtraInfo, LogicalType::STRING(size_t max_length), and YAML parsing for string.var_char.max_length (see g_type_utils.h, types.h, modern_schema.yaml). This request is mainly to add explicit VARCHAR(max_length) support and consistent exposure at the type system’s external interface.