Adds support for letter spacing

[kennethormandy]

Description

This pull request adds support for letter spacing, via the growing support for the canvas letter spacing property, and the existing support in SVG.

• SVG letter-spacing: ~96% browser support
• Canvas letterSpacing: ~76% browser support (not supported in Safari)

All browser tests are passing, including some new ones for this feature.

Example

var text = new PointText({
  fontFamily: 'Charter',
  fillColor: 'black',
  content: 'Paper.js',
  // Etc.

  letterSpacing: 10,
});

API considerations

I’d be interested in getting your feedback on a couple of possible adjustments, @lehni:

• Number only? Should letterSpacing only accept a number that’s intended to be relative to the font size, like 0.2, rather than a value with units, like '10px'? Ie. Treat all values like ems? This appears to be what OpenType.js does, and it feels like the better way to use it to me.
• Tracking or Letter Spacing Depending on that, should it actually be called tracking, and use integers? I tend to prefer the CSS-like names, but it feels a bit like leading and justification in that tracking would be the more expected name within Paper.js.

Related issues

• None that I could find

Checklist

• New tests added or existing tests modified to cover all changes
• Code conforms with the JSHint rules (yarn run jshint passes)

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Thanks for considering this, and for all the great work on this library over the years!