Switch to using pako from node's zlib

[matthewturk]

In #319, zlib was used to decompress gpx.gz files. This switches to using pako, which is a pure-JS library that doesn't rely on Node's zlib linkings.

The two things I was not certain about:

• I added the dependency to the dependencies list in package.json. Does it need to be anywhere else?
• I've changed to using non-promisify'd ungzip, as the output values were not being correctly passed through the promisification when I used the options. (I was ending with Promise<unknown> as the return value). Any suggestions there would be helpful!

This should close #325.

[valentine195]

Thank you, just tested on mobile and it does indeed load.