Bump minimum target to es2015

[SvanT]

Now that we no longer support IE we can raise the minimum ES target a
bit. Changing to es2015 shaves off ~17% from the main bundle size, also
it prevents generators from being transpiled into regular functions etc.
so I would guess it has the potential to give some general performance
improvements too.

For fun I also tried to raise it to es2022, this gave a slightly bigger
bundle than es2015 so I think es2015 could be an ok target unless we
need something newer for some reason.

[Tyriar]

17% is a pretty awesome win, I'll merge in and try update it in vscode soon to see if anything goes wrong 🤞

[jerch]

Yes the savings are awesome 👍

On a sidenote: I wonder if ts compiler/webpack could be instructed to mangle private names as well (with the help of a skip list in case we really do ugly private access from outside of a module/class). When looking at the generated output it really seems, that most of the package size results from chatty property names, that dont get mangled. (Rough estimate from a python script extracting chatty names sees this at 60%, lol)

[Tyriar]

I don't think the TS compiler can do it, but I'd love to minify some of those long names without compromising readability/sourcemaps.

Before:

npm notice 323.4kB lib/xterm.js 
npm notice 102.5kB lib/xterm-addon-webgl.js 

After:

npm notice 268.6kB lib/xterm.js
npm notice 85.2kB  lib/xterm-addon-webgl.js