spec/lex.dd: Specify DelimitedString mandatory line breaks in grammar

[CyberShadow]

This implements a suggestion from a previous PR: https://github.com/dlang/dlang.org/pull/3033#discussion_r663481518

We can't describe (within the grammar) that Delimiter and MatchingDelimiter must match, nor that they can't appear within the line, but this brings us a iota closer towards accuracy.

[dlang-bot]

Thanks for your pull request, @CyberShadow!

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[dkorpel]

We can't describe (within the grammar) that Delimiter and MatchingDelimiter must match, nor that they can't appear within the line, but this brings us a iota closer towards accuracy.

We also might need to figure out how to exclude delimiter characters (newline in this case) from WysiwygCharacter

[PetarKirov]

@CyberShadow please rebase.