Support Apple Silicon BrowserStackLocal binary

[robcrocombe]

The only MacOS option for downloading a binary of BrowserStackLocal is darwin-x64 which is for Intel Macs. This does not work on Apple Silicon Macs, which have superseded Intel Macs for the last 5 years. Please build an Apple Silicon version of your binary and update the references in the code and in the website documentation, neither of which even mention this constraint. Thanks!