Add RawMessage.Type to report a value's CBOR major type

[ChrisJr404]

Description

This adds a Type method to RawMessage so callers can check the outer kind of a raw value without fully decoding it, which is what #440 asked for.

The motivation is the same one from that issue: with json.RawMessage you can peek at the first byte to see whether you're looking at an object, array, string, etc., but doing the equivalent with a cbor.RawMessage meant writing a little partial decoder over the initial byte and remembering the CBOR major type encoding. RawMessage.Type() reads the major type from the initial byte and returns it as an exported Type value, so branching on the kind is a one-liner:

switch t, _ := raw.Type(); t {
case cbor.TypeArray:
    // ...
case cbor.TypeMap:
    // ...
}

The exported Type mirrors the major types in RFC 8949 Section 3.1, and its String() reuses the descriptions the package already had internally so there's no second copy of that mapping to keep in sync. Type() only looks at the initial byte and deliberately does not check well-formedness, so it stays cheap and has no effect on anyone not calling it; callers that need validation can still use Wellformed. It returns an error when the message is empty.

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue feature: inspect first/outer "kind" without full decode #440
• Closes or Updates Issue feature: inspect first/outer "kind" without full decode #440

Checklist (for code PR only, ignore for docs PR)

• Include unit tests that cover the new code
• Pass all unit tests
• Pass all lint checks in CI (goimports, gosec, staticcheck, etc.)
• Sign each commit with your real name and email.
• Certify the Developer's Certificate of Origin 1.1

Certify the Developer's Certificate of Origin 1.1

• By marking this item as completed, I certify the Developer Certificate of Origin 1.1.