Enhancement: Generate specialized mergePartial() method when optimize_for = SPEED

[jcready]

Currently specialized methods are generated for: create(), internalBinaryRead(), and internalBinaryWrite() when optimize_for = SPEED

protobuf-ts/packages/plugin/src/code-gen/message-type-generator.ts

Lines 116 to 123 in 3a7ce47

	// "MyMessage$Type" members for optimized binary format
	if (optimizeFor === OptimizeMode.SPEED) {
	classDecMembers.push(
	...this.typeMethodCreate.make(source, descriptor),
	...this.typeMethodInternalBinaryRead.make(source, descriptor),
	...this.typeMethodInternalBinaryWrite.make(source, descriptor),
	);
	}

The same could be done for mergePartial() to avoid relying on runtime reflection and iteration.