update eslint rules to prefer single quotes

[JustinBeckwith]

I was reading a bit of PR feedback over here, where the developer used backticks over single quotes, when not strictly necessary:
https://github.com/googleapis/nodejs-translate/pull/234/files#r271541085

If this is our story (as I think it should be), we should update our default eslint rules to enforce this via the linter:
https://eslint.org/docs/rules/quotes

[bcoe]

@JustinBeckwith can we address this with google/gts#421

[JustinBeckwith]

Of course!

[bcoe]

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