Add line_length parameter: ignores_multiline_strings

[thisIsTheFoxe]

• Rule Request: [LineLengthConfiguration - Add ignoring rule for multiline strings]  #2689
• ignore swift multiline strings marked by """

The idea here is:

1. if line is a string only (within two """)
2. and if next line that isn't a string only doesn't start with """ -> ignore as it's part of a multiline string
   if 2 is false, then we might be within a string that's concatinated:

let a = "abc" +
"def"
+ "ghi"

[SwiftLintBot]

	1 Warning
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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Here's an example of your CHANGELOG entry:

* Add `line_length` parameter: `ignores_multiline_strings`.  
  [thisIsTheFoxe](https://github.com/thisIsTheFoxe)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

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