Convert if to switch statement

[jstashh]

Remove the if statements from this block of code - https://github.com/yearn/yearn-sdk/blob/master/src/interfaces/token.ts#L67-L79

Use a switch instead, as will be easier to add future chains

[MahmoudHamdy02]

I would like to work on this as my first contribution!

[velvet-shark]

Submitted a pull request for this.

[jstashh]

Done in https://github.com/yearn/yearn-sdk/pull/234