Add TextEncoder and TextDecoder TypeScript type declarations

[deyaaeldeen]

Hey! Both TextEncoder and TextDecoder are now natively available in Hermes:

• TextEncoder landed in Hermes for RN 0.74 (using TextEncoder with hermes hermes#948)
• TextDecoder landed in Dec 2025 (Add support for TextDecoder hermes#1855), should ship with RN 0.85

But neither is declared in React Native's TypeScript globals (src/types/globals.d.ts), so TypeScript users have to either cast through any, pull in lib: ["dom"], or add their own declarations to use these APIs without type errors.

This adds type declarations for both APIs to globals.d.ts, following the same interface + var pattern used by all other Web API types in the file. The types match what Hermes actually implements:

TextEncoder:

• encoding (always "utf-8")
• encode(input?): Uint8Array
• encodeInto(input, destination): { read, written }

TextDecoder:

• Constructor accepts an optional encoding label and { fatal?, ignoreBOM? } options
• encoding, fatal, ignoreBOM getters
• decode(input?, { stream? }): string
• Supported encodings: UTF-8, UTF-16LE/BE, and single-byte encodings (ISO-8859-, Windows-, KOI8-*, IBM866, Macintosh, x-mac-cyrillic)

Test Plan

Added type tests in packages/react-native/types/__typetests__/globals.tsx covering:

• TextEncoder: constructor, encode(), encode(string), encodeInto(), and encoding literal type
• TextDecoder: constructor (no args, with label, with options), decode() from Uint8Array/ArrayBuffer/empty, decode with { stream } option, and all three getters (encoding, fatal, ignoreBOM)

Ran tsc --noEmit against the type tests — no new errors introduced.

Changelog:

[General] [Added] - Add TypeScript type declarations for TextEncoder and TextDecoder

Closes #56325

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