Move new format addons into repo, remove old addons

[Tyriar]

Fixes #2142
Fixes #2161

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Notes

• The addons are built like they were before using tsconfig.all.json, they have been moved out of src however to further disassociate them with the core code
• The demo uses the addons in a very similar way to how they would when using the module from npm.
• The addon folders don't get their dependencies installed, instead they rely on the main node_modules and only have a single script for publish which calls ../../../node_modules/.bin/tsc directly
• The addons are scoped to @xtermjs as this is what it will be when we use the GH package registry
• I archived the repos xterm-addon-attach, xterm-addon-search and xterm-addon-web-links
• Removed the addon label and added separate area labels for each addon

[mofux]

Works great and LGTM 👍. Does it make sense to add some words about how the addons work and how they share their dependencies with the root project to our wiki?

[Tyriar]

@mofux good idea, I was also thinking it might be good to detail the #1507 wip in the wiki as things will probably be in flux for quite a while still.

[Tyriar]

Updated the wiki