Closes denoland#5831 (for the `@std/cbor` module).

`encodeCbor` / `encodeCborSequence` / `CborSequenceEncoderStream` /
`CborArrayEncoderStream` never mutate their inputs, but the
`CborType` parameter required mutable arrays, a mutable `Map`, and a
mutable index signature. That meant `as const` literals, `ReadonlyMap`
values, and `readonly T[]` arrays could not be passed without a cast,
even though the runtime behaviour is identical.

Add a new input-only type `CborInputType` that mirrors `CborType` but
substitutes:

- `CborType[]`               -> `readonly CborInputType[]`
- `Map<CborType, CborType>`  -> `ReadonlyMap<CborInputType, CborInputType>`
- `{ [k: string]: CborType }`  -> `{ readonly [k: string]: CborInputType }`

`CborType` (which is also the decoder output type) stays mutable, so
this is fully backwards-compatible for existing callers and for the
decode side. `CborStreamInput` is input-only and is widened in place.

In the encoders, the existing `x instanceof Map` narrowing does not
narrow `ReadonlyMap` on its own, so a tiny `isReadonlyMap` type
predicate is added in `_common_encode.ts` and the relevant
`#encodeArray` / `#encodeObject` stream helpers are widened to
accept the readonly variants.

`CborTag<T>`'s constraint is widened from
`T extends CborType | CborStreamInput | CborStreamOutput` to also
permit `CborInputType`.

Mirrors the `@std/msgpack` change from denoland#5832.

Regression tests added:

- `encodeCbor()` accepts deeply-readonly `as const` literals.
- `encodeCbor()` accepts readonly tuple literals.
- `encodeCbor()` accepts `ReadonlyMap` input.
- `encodeCbor()` accepts `{ readonly [k: string]: ... }` input.
- `encodeCbor()` accepts a `CborTag` whose content is readonly.
- `encodeCborSequence()` accepts a `readonly` array of values.

All 89 CBOR module tests pass (24 pre-existing + 65 from related files;
6 new). `deno check cbor/` clean. `deno lint cbor/` clean.
`deno fmt --check cbor/` clean.

…ments

Per @BlackAsLight on denoland#7148:

- Rename CborInputType -> ReadonlyCborType. Encoder signature now
  reads encodeCbor(value: CborType | ReadonlyCborType): Uint8Array
  (and the equivalent in encodeCborSequence), which keeps CborType
  prominent in the public API and clearly documents what additional
  shapes the encoder accepts.
- Drop the descriptive comments on the new tests. The test names
  already describe what they cover, and the comments would lose
  context over time.

bartlomieju noted on denoland#7148 that the union looks redundant at first
glance because mutable Map/array/index-signature are all assignable
to their readonly counterparts. The reason it is needed is that
CborTag<T> has a writable tagContent field and is therefore
invariant in T, so CborTag<CborType> is not assignable to
CborTag<ReadonlyCborType> and the union preserves existing CborTag
callers. Document that on ReadonlyCborType so the union is not
'simplified' away later.