Add check for "dotlines" with cabal 3.0+

[tbidne]

Adds a warning for #11518. I'm not too familiar with this part of the codebase, but the changes seem straightforward.

Essentially wraps

fromUTF8BS :: ByteString -> String

with

parseLine3 :: Position -> BS.ByteString -> ParseResult src String

which adds a warning if the string matches the same dot check pre-3.0 (trim bs == "."). The rest is just mechanical changes to accommodate the ParseResult.

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[philderbeast]

With -XViewPatterns, you could skip binding bs. Bindings for s' and msg could also be skipped.

Suggested change

	parseLine3 p bs = do
	when (s' == ".") $ parseWarning p PWTDotline msg
	pure s
	where
	s = fromUTF8BS bs
	s' = trim s
	msg =
	"Empty lines with a dot '.' are unnecessary for creating newlines "
	++ "(and treated literally) in cabal 3.0+"
	parseLine3 p (fromUTF8BS -> s) = do
	when (trim s == ".") . parseWarning p PWTDotline $
	"Empty lines with a dot '.' are unnecessary for creating newlines "
	++ "(and treated literally) in cabal 3.0+"
	pure s

[tbidne]

Reasonable, though I think turning on ViewPatterns for the whole module is overkill, just for this. I don't feel strongly though.