Update safe-string-coercion to handle additions of string literals

[poteto]

Adding strings shouldn't trigger a lint violation of this rule, since adding strings are always safe.

[sizebot]

Comparing: aed33a4...4d32d44

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	135.46 kB	135.46 kB	=	43.41 kB	43.41 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	147.72 kB	147.72 kB	=	47.17 kB	47.17 kB
facebook-www/ReactDOM-prod.classic.js	=	491.70 kB	491.70 kB	=	87.49 kB	87.49 kB
facebook-www/ReactDOM-prod.modern.js	=	477.00 kB	477.00 kB	=	85.24 kB	85.24 kB
facebook-www/ReactDOMForked-prod.classic.js	=	491.70 kB	491.70 kB	=	87.49 kB	87.49 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 4d32d44

[josephsavona]

I think you need a check that the operator is +?

[poteto]

@acdlite would appreciate a review if you have time, not a big deal but solves a minor annoyance (typing 'my long message that spans ' + '' gives you an immediate red squiggly)

[sophiebits]

To confirm, this is just annoying while typing in development right? There's no need to commit a + '' with a string literal.

Worth checking that the template literal doesn't have any substitutions?

[sophiebits]

I guess even if the template literal has substitutions then the + '' is fine since it's already a string at that point. We might want to lint on the template literal itself but not the concatenation. However a tagged template literal could be a non-string I think.

[poteto]

@sophiebits Yeah this is really a mild annoyance at dev time when you're say, writing a long error message that spans multiple lines. Good point about tagged template literals returning non-strings though, I just tried it out and it looks like it is possible. I'll see if I can just match on non-tagged

[poteto]

Updated so we only treat untagged template literals as strings.