restrict-plus-operands: Minor error message change.

[bowenni]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Suggest template literals when an operand is a string.
Most violations in Google code base is "string + number". Users might do .toString() to pass this check if they didn't know about template literals.

Is there anything you'd like reviewers to focus on?

CHANGELOG.md entry:

[enhancement] restrict-plus-operands: More specific error message when arguments include strings

[palantirtech]

Thanks for your interest in palantir/tslint, @bowenni! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bowenni]

I am covered by the Google corporate CLA.