Primarily because it's been ages since we bumped eslint and as a precursor
to allowing some future test files to be written in a more modern type
of JavaScript. Especially think about tests written using puppeteer.js
which is heavily `Promise` based, where the async-await syntax makes
it soooo less painful to write.

Silly not being able to use that because of an outdated eslint version
doesn't understand that syntax construct at all.

These changes fixes an indentation bug introduced when partials was
made to also be indented like its parent template. When partials are
rendered inline, it should not be indented with the amount of indentation
that the line already has.

Fixes https://github.com/janl/mustache.js/issues/715
Refs https://github.com/janl/mustache.js/pull/705