Pass optional `postscriptName` parameter to fontkit

[tsung-ju]

Currently passing .ttc files to fromFile, fromBlob or fromUrl causes an error, because fontkit returns a Collection instead of a Font. I added an optional postscriptName parameter which tells fontkit to return a specific font from the collection.

Here's an example usage:

const metrics = await fromFile('NotoSans.ttc', 'NotoSans');