Add struct field tag support for JSON customization

Summary

Support struct field tags/attributes for customizing JSON field names and behavior, similar to Go's struct tags.

Background

Go uses struct tags like `json:"fieldName,omitempty,string"` to customize JSON behavior per-field.

In Zig, we can use doc comments or a convention-based approach since Zig doesn't have runtime tags.

Proposed Approaches

Option 1: Comptime Field Configuration

const Person = struct {
    first_name: []const u8,
    age: ?u32,
    _private: u32,  // Fields starting with _ are skipped
    
    pub const json_config = .{
        .first_name = .{ .name = "firstName" },
        .age = .{ .omitempty = true },
    };
};

Option 2: Wrapper Types

const Person = struct {
    first_name: json.Field([]const u8, .{ .name = "firstName" }),
    age: json.Optional(u32, .{ .omitempty = true }),
    secret: json.Skip([]const u8),
};

Features Required

• Custom field names (name)
• Omit empty/zero values (omitempty, omitzero)
• Skip field entirely (-)
• Stringify numbers as strings (string)
• Inline/flatten nested structs (inline)
• Format specifiers (format:base64, format:unix)
• Case sensitivity options (case:ignore, case:strict)

Reference

Go struct tags: json:"name,omitempty,string,inline,format:base64"

Priority

High - Essential for API compatibility