Build using TypeScript project references

[Tyriar]

The performance of project references is at a level where we can actually use them now, this change vastly simplifies out build process by essentially having a "solution" file whose job it is to build the other sub-projects (xterm.js and the addons). The dependencies look like this:

src/tsconfig.all.json
  src/tsconfig.json
    src/core/tsconfig.json
      src/common/tsconfig.json
    src/common/tsconfig.json
  src/addons/attach/tsconfig.json
  src/addons/...

The vision here is that once the layering refactor is complete for it to look like this:

src/tsconfig.json (does nothing but links to projects to build)
  src/public/tsconfig.json
    src/ui/tsconfig.json
      src/common/tsconfig.json
    src/core/tsconfig.json
      src/common/tsconfig.json
    src/common/tsconfig.json
  src/addons/attach/tsconfig.json
  src/addons/...

Other things this PR does:

• Updates TS to 3.4
• Tweaks how the CSS build step works and removes our need for concurrently which will fix yarn is eating following commands #1639, part of this was moving the demo to depend on src/ directly instead of build/
• Removed the need for the layering step, strict checks for core/common are performed as part of the regular watch
• Removes addon watch, there is a single yarn watch which will also watch for changes in addons and update that project only if a file change occurs
• Removed the zmodem demo, no one maintains it and we'll be removing the zmodem addon with the upcoming layering/addon changes
• There is a base and a library-base tsconfig which other tsconfigs extend in order to reduce duplication

Fixes #1337
Fixes #1639