Updated TextGeometry to use ES6 class format.

[jmschrack]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Feature (non-breaking change which adds functionality)
• [ ] Code style update
• [x] Refactor (refactoring or adding test which isn't a fix or add a feature)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

Did you test your solution?

• [ ] I lightly tested it in one browser
• [x] I deeply tested it in several browsers
• [ ] I wrote tests around it (unit tests, integration tests, E2E tests)

I tested all the provided samples in Edge, Chrome, and Firefox. They all worked fine. 👍

Problem Description

Three-bmfont-text is incompatible with ES6 classes. See: https://github.com/Jam3/three-bmfont-text/issues/46

Solution Description

var Base = THREE.BufferGeometry
function TextGeometry (opt) {
  Base.call(this)
...
}
inherits(TextGeometry, Base);

was changed to

class TextGeometry extends THREE.BufferGeometry {
  constructor(opt){
    super(opt);
...
  }
}

Side Effects, Risks, Impact

• [x] N/A

Aditional comments:

[jmschrack]

Accidentally made this from my master branch instead of my dev branch. Closing and reopening as https://github.com/Jam3/three-bmfont-text/pull/49