Readonly exotic arrays

[danielnixon]

Readonly versions of all of these:

interface Int8Array extends RelativeIndexable<number> {}
interface Uint8Array extends RelativeIndexable<number> {}
interface Uint8ClampedArray extends RelativeIndexable<number> {}
interface Int16Array extends RelativeIndexable<number> {}
interface Uint16Array extends RelativeIndexable<number> {}
interface Int32Array extends RelativeIndexable<number> {}
interface Uint32Array extends RelativeIndexable<number> {}
interface Float32Array extends RelativeIndexable<number> {}
interface Float64Array extends RelativeIndexable<number> {}
interface BigInt64Array extends RelativeIndexable<bigint> {}
interface BigUint64Array extends RelativeIndexable<bigint> {}