Add `es2026` as valid `target` and `lib`

[DanielRosenwasser]

To quote the spec:

ECMAScript 2026, the 17th edition, added Math.sumPrecise for summing an iterable of Numbers of varying magnitude while minimizing precision loss; Iterator.concat for sequencing iterators; Array.fromAsync for constructing Arrays from async iterables and other async sources; Error.isError for identifying error objects; methods to Map.prototype and WeakMap.prototype for providing a default value to use during retrieval when a key is not already present; methods to Uint8Array for converting to and from Strings of hexadecimal- and base64-encoded binary data; a parameter to JSON.parse revivers to access the matched segment of JSON source; and JSON.rawJSON for fine control over JSON.stringify output for primitive values.

We need to add the following:

• Math.sumPrecise
• Array.fromAsync
• Iterator.concat
• JSON.rawJSON
• An overload(?) for JSON.stringify

The following must be moved to an es2026 lib:

• Error.isError
• Map.prototype.*
  • getOrInsert
  • getOrInsertComputed
• WeakMap.prototype.*
  • getOrInsert
  • getOrInsertComputed
• Uint8Array.prototype.*
  • toBase64
  • setFromBase64
  • toHex
  • setFromHex
• Uint8Array.fromHex
• Uint8Array.fromBase64