Add platform fallback implementation

[simonbrunel]

If window or document are undefined, a minimal platform implementation is used instead, which one only returns a context2d read from the given canvas/context.

Relates to #4591

[simonbrunel]

@hach-que can you please test this PR and confirm that it works for your use case?

[simonbrunel]

@hach-que ping!

[hach-que]

Probably won't get a chance to test this for a while. As long as it avoids calling methods on window or document when they're not available, it should be fine.

[simonbrunel]

I can't test it server side so can't guarantee that it will work as expected. Basically, if window and document are undefined, then we use the default basic implementation which contains no reference to window or document (ie. no responsive resize or chart events).

[hach-que]

Found 5 minutes to test this. The following works:

• yarn init
• yarn add https://github.com/simonbrunel/Chart.js#platform-basic
• Write an index.js file like:

var cjs = require('chart.js');

console.log('library loaded successfully under Node.js!');

var c = new cjs();

console.log('made a chart:');
console.log(c);

• Run node index.js.
• You'll get the following successful output:

library loaded successfully under Node.js!
Failed to create chart: can't acquire context from the given item
made a chart:
Chart {
  id: 0,
  ctx: null,
  canvas: null,
  ... snipped ... }

[simonbrunel]

Thanks @hach-que