Switch the examples back to mainline xterm.js, pending manual box char drawing

[rrbutani]

As mentioned in 62d38dc, we're using a fork that has a patch (xtermjs/xterm.js@master...guregu:box-drawing from xtermjs/xterm.js#2409 (comment)) that renders these characters correctly.

Once xtermjs/xterm.js#2409 is resolved, we should switch the examples back to using mainline xterm.js.

xterm-js-sys/examples/package.json

Lines 13 to 14 in 62d38dc

"//": "switch back when xtermjs/xterm.js#2409 gets resolved: 'xterm': '^4.7.0'",

"xterm": "https://pkg.githubusercontent.com/272294459/576f5500-ae67-11ea-99ba-6a68eeed3a23?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20200614%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20200614T225631Z&X-Amz-Expires=300&X-Amz-Signature=3e453570a46130497bad93ca6dcc282b24c6fcc5739f845384c12ddcb193cab7&X-Amz-SignedHeaders=host&actor_id=0&repo_id=0&response-content-disposition=filename%3Dxterm-4.7.0-npm.tgz&response-content-type=application%2Foctet-stream",