Create a node.js target for xterm.js

[joyceerhl]

Hey @Tyriar, here's an attempt at what you described in #2749 (comment). This PR:

• Duplicates src/browser/public/Terminal.ts --> src/common/public/Terminal.ts.
• Duplicates src/browser/Terminal.ts --> src/common/public/TerminalCore.ts since src/browser/public/Terminal.ts wraps src/browser/Terminal.ts.
• Removes DOM/UI references and src/browser imports from src/common/public/Terminal.ts and src/common/public/TerminalCore.ts. I took a guess at what to remove based on what I thought would still be meaningful in the Node environment and on the discussion in Create a node.js target for xterm.js #2749, but I'd appreciate feedback here.
• Adds a separate webpack config and tsconfig as well as a small 'testcase', node-test/index.js, which I used to verify that the changes described above can be compiled, built, and used in a Node project based on the included README.
• I only checked in xterm-core.d.ts in the first commit in this PR to get the project compiling, since I expect that I've either taken out too much or not enough from src/common/public/Terminal.ts. Changes to that file should be disregarded for now.

I'm expecting this to need lots of work—looking forward to your feedback!

[Tyriar]

Nice work, some high level comments below 🙂

[Tyriar]

This seems like a problem, we can't import from xterm if we want it to run for node because it touches DOM types which will throw for tsconfig targets without the dom lib.

[Tyriar]

Another problem I'd like for us to solve is the sharing of these typings, I think having xterm.d.ts import from xterm-core.d.ts will cause problems because of this module declaration. We may need separate .d.ts that just export the core types without the module wrapping them to be able to share.