feat!: prefer single quotes if possible, raising warning

[bcoe]

Warn if single quotes are not used in a situation in which this is possible. see: https://eslint.org/docs/rules/quotes

This rule lets us catch the behavior discussed here, in which a template string is used with no replacements.

I've opted for a warning rather than an error, as in testing it seemed like this rule could be a bit disruptive for folks who write strings like this:

"hello 'world'"

as it requires the replacement:

'hello "world"'

fixes: googleapis/google-cloud-node#2915

[codecov-io]

Codecov Report

Merging #475 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #475   +/-   ##
=======================================
  Coverage   96.33%   96.33%           
=======================================
  Files           6        6           
  Lines        1118     1118           
  Branches      153      153           
=======================================
  Hits         1077     1077           
  Misses         41       41           

Impacted Files	Coverage Δ
src/clean.ts	100.00% <100.00%> (ø)
src/init.ts	88.07% <100.00%> (ø)
test/test-init.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f9b20a6...3f7db35. Read the comment docs.