June plan for xterm.js in the terminal

[Tyriar]

A lot of things have been happening in xterm.js with the upcoming xterm v4 that we need to adopt as part of the June release. Here are the highlights:

• Old addon support is dropped, only new addons (added in 3.14) will work. These new addons support TypeScript much better and have a much better thought out lifecycle than the previous model (Move new format addons into repo, remove old addons xtermjs/xterm.js#2164)
• Adopting the new buffer and selection APIs allows us to remove some of the private API typings we touch (Buffer API xtermjs/xterm.js#2074, select and getSelectionPosition APIs xtermjs/xterm.js#2078)
• New addons are published as separate npm modules which allows us to remove the dodgy __nodeRequire calls (Avoid __nodeRequire() #74398)
• Since beta builds in xterm.js are now automated we can move over to use the official xterm module instead of vscode-xterm (Should we move to official xterm and how to deal with problem of new addon peer dependencies #74690)
• The working with xterm.js wiki article should be updated to reflect these changes
• The xterm module main JS file is now webpacked so which makes it a single file and minified (Enable absolute imports and minify distributable xtermjs/xterm.js#2132)
• Continued improvement to the layering and enabling TS strict mode makes the library more stable Improved Layering and enabling TS strict xtermjs/xterm.js#1507