mixin composition + string split cache util (fixes #3270)

[ngokevin]

Description:

Allow for mixin composition to build "classes" out of multiple mixins.

<a-mixin id="font" text="font: myfont.json"></a-mixin>
<a-mixin id="red" text="color: red"></a-mixin>

<a-mixin id="redText" mixin="red font"></a-mixin>

<a-entity mixin="redText someMixin">
// This will turn into `<a-entity mixin="red font redFont someMixin">`

Changes proposed:

• Allow for mixin composition.
• String split util (used by mixin) which caches string split operations to save memory.

[ngokevin]

#3302 is relevant for making sure defined attributes don't get overridden by mixins.

[dmarcos]

Can we write some tests for this?

[ngokevin]

Yeah, will do

[ngokevin]

Updated.

[dmarcos]

Tests failing on Firefox

[ngokevin]

Updated