Fix readJSFiles

[wagerfield]

Since 3.x specifying string paths for workboxExtensions, cachingExtensions and routingExtensions as per the docs causes an error to be thrown.

The docs say that these fields should be a String that resolves to a file to load and inject into the sw.js template:

https://pwa.nuxtjs.org/modules/workbox.html#workboxextensions

...however, the new implementation of readJSFiles only supports String[].

The readJSFiles util that is called for each of the extensions fields uses Array.from(files). When files is a String it gets split into characters and the resolver borks since it cannot find a file.

This PR adds a check to readJSFiles for string values and wraps them in an array—making it backwards compatible.

I have also updated the workbox docs to reflect this.

[codecov]

Codecov Report

Merging #143 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
##            dev   #143   +/-   ##
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  Coverage   100%   100%           
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  Files         1      1           
  Lines         2      2           
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  Hits          2      2

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[pi0]

Thanks 😊

[pi0]

I suggest using Array.isArray here

[wagerfield]

Done 👍