Hunk.BlockTag should be called InfoString

[warpfork]

Apparently, the text after the characters indicating the start of a code block are called the "info string": https://spec.commonmark.org/0.30/#info-string

Currently the go-testmark code called it "BlockTag":

go-testmark/testmark.go

Lines 49 to 51 in 53baa42

	// The code block syntax hint (or more literally: anything that comes after the triple-tick that starts the code block).
	// Usually we don't encourage use of this much in testmark, but it's here. Can be empty.
	BlockTag string

I like my name better, of course (I'm pretty sure someone would know what you mean faster when you say "the code block tag" than "the info string" until you bang them over the head with the spec link), but, ah well. It was a quick choice made without research.

Might as well follow the upstream lexicon as long as there is indeed a clear name specified for it.