feat: ex.FontSource resource type

[mattjennings]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

==================

Closes #

Changes:

• Added ex.FontSource resource type

  const fontSource = new ex.FontSource('/my-font.ttf', 'My Font')
  loader.addResource(fontSource)
  
  game.start(loader).then(() => {
    const font = fontSource.toFont() // returns ex.Font
  })

  Font options can be defined either at the source or at the toFont() call. If defined in both, toFont(options) will
  override the options in the FontSource.

  const fontSource = new ex.FontSource('/my-font.ttf', 'My Font', { 
    filtering: ex.ImageFiltering.Pixel,
    size: 16, // set a default size
  })
  const font = fontSource.toFont({
    // override just the size
    size: 20,
  })

• adds "dom.iterable" to lib in tsconfig.json as it was needed to get the correct types for document.fonts

[eonarheim]

@mattjennings Looks perfect! Only lint is failing

[mattjennings]

oops! for some reason my IDE wasn't showing anything. Just pushed a fix.

[eonarheim]

Awesome, will merge on green 😎

[eonarheim]

Thanks again @mattjennings!