Focus TextViewHighlighter on character changes

[DivineDominion]

NSTextStorageDelegate is informed about attribute changes, and the old setup would apply highlighting to attribute-only changes, too.

That doesn't sound too bad unless you begin to store attribute in the text storage:

Neon/Sources/Neon/TextViewSystemInterface.swift

Lines 72 to 74 in 153d6e0

	// finally, fall back to applying color directly to the storage
	assert(textStorage != nil, "TextView's NSTextStorage cannot be nil")
	textStorage?.setAttributes(attrs, range: clampedRange)

because then the highlighter loop will never stop.

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Why I noticed and why I believe this is actually the correct way, stay tuned for Follow Up PR 3 :)

[DivineDominion]

@mattmassicotte Sorry, forgot to mention I went ahead and made executionMode injectable (required in part to test this, but then also to use it in .synchronous mode in my app)

[mattmassicotte]

Great change, thank you!