indentation_width should exclude indentation within multiline strings

[robertlacroix]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

indentation_width probably shouldn't flag content in multiline strings, as it's likely not Swift code and might follow different indentation norms.

Complete output when running SwiftLint, including the stack trace and command used

$ swiftlint lint test.swift --config indent.yml
Linting Swift files at paths test.swift
Linting 'test.swift' (1/1)
test.swift:3:1: warning: Indentation Width Violation: Code should be indented using one tab or 4 spaces. (indentation_width)
Done linting! Found 1 violation, 0 serious in 1 file.

$ cat test.swift
let jsonFixture = """
{
  "test": true
}
"""

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.49.1
• Installation method used (Homebrew, CocoaPods, building from source, etc)? building from source
• Paste your configuration file:

opt_in_rules:
  indentation_width