Allow using different port

[dufferzafar]

Currently port is hardcoded to 5600. Add a setting for the same.

Similar to ActivityWatch/aw-watcher-web#79

[dufferzafar]

I installed the extension from VSCode Marketplace then went ahead and made the changes in the VScode extension directory:

$ pwd
C:\Users\duffe\.vscode-insiders\extensions\activitywatch.aw-watcher-vscode-0.5.0

$ rg -uu 5700
out\aw-client-js\src\aw-client.js
20:            const port = !options.testing ? 5700 : 5666;

And it worked!