Correct parser translator ast for regex with line continuation or empty regex

[Earlopain]

Two ast fixes for regex nodes:

• When the regex is empty (//) the parser gem doesn't emit a string node
• When the regex contains line continuations, apply the same logic as was done for heredocs Fix parser translator ast for heredoc with written newlines #3347. This looks ok to share.

Actually, the same line continuation logic should probably also be applied to string nodes. There's even a comment for that:

prism/lib/prism/translation/parser/compiler.rb

Lines 1686 to 1690 in 29f2a06

	# This occurs when we have line continuations in the string. We
	# need to come back and fix this, but for now this stops the
	# code from breaking when we encounter it because of trying to
	# transpose arrays of different lengths.
	[builder.string_internal([node.unescaped, srange(node.content_loc)])]

The ast looks correct but no new tests pass so I can't confidently say nothing else breaks. I'll come back to this later once I hammered out a few more failures.