refactor(@angular-devkit/core): deprecate stringToFileBuffer and fileBufferToString

DEPRECATED: `stringToFileBuffer` and `fileBufferToString` are deprecated. Use standard Web APIs (`TextEncoder` and `TextDecoder`) instead.

Internal usages within the repository have been removed and replaced with standard Web APIs. The public API golden file for `@angular-devkit/core` has been updated to reflect the deprecations.