Prepare JS Modules to be more extendable

[myyk]

This is a very opinionated change which is inspired by my experiment with running NextJS on Mill in https://github.com/myyk/mill/tree/example-nextjs. It would be a smaller diff to merge that build off of this.

This change adds these features:

• rename TypeScriptModule to TscModule since there will be many types of TypeScript projects that wouldn't even want to extend it.
• Split out the node package managing stuff into NodeModule. In the future, this might even want to have flavors of NpmModule/YarnModule/etc but I don't want to over engineer this.
• install is a new command which can be more agnostic to npm or yarn or whatever and will install deps and copy
  node_modules/ to the user's project outside of the cache to help the IDE. This will make auto-complete work a lot of the time.
• Splits out dev and normal dependencies like would be done in a node project.
• Ignores node_modules/ in mill repo
• Adds ignoring node_modules/ in examples, assuming user can copy example into their repo as a template.

Future work after this:

• Add a NextJsModule which extends NodeModule and looks a lot like this: https://github.com/myyk/mill/blob/example-nextjs/example/javascriptlib/basic/2-nextjs/build.mill

[lihaoyi]

Let's merge #4003 before we proceed with this one, since that one has been WIP for a while and seems almost done