Consider to implement ReaderJsonLexer::peekLeadingMatchingValue?

[iseki0]

What is your use-case and why do you need this feature?
Polymorphic deserialization on stream also expects a well performance.

Describe the solution you'd like
Would using InputStream#mark and InputStream#reset to implement this method be a good solution?

kotlinx.serialization/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/ReaderJsonLexer.kt

Lines 178 to 180 in d0ae697

	// Can be carefully implemented but postponed for now
	override fun peekLeadingMatchingValue(keyToMatch: String, isLenient: Boolean): String? = null