Migrate javascript files to typescript

[yufengwng]

TypeScript and JavaScript can co-exist, but for sake of development we should probably just stick to one thing. TypeScript has a number of benefits, and vscode extensions are typically written in typescript. Convert all the js files we have to ts.

[yufengwng]

This is not strictly necessary, just a nice-to-have. It'll be a lot of effort though. Can be a low-priority.

[yufengwng]

As part of #19, the webpack config has been updated to support typescript, so we can use typescript code in the javascript code. This way, we can gradually migrate the javascript to typescript.