Upgrade web client and replace deprecated npm packages

[Jo-Blade]

This merge request follows #56 because I've noticed the web-client does not work anymore with latest webtty and that some npm packages are deprecated.

It has been tested to work on my nixos computer

What has been done:

• upgrade all npm dependencies
• copied latest "wasm_exec.js" file from go source code
• migrate from deprecated "parcel V1" to latest parcel (see https://parceljs.org/migration/parcel-1/)
• replace deprecated "xterm" by "@xterm/xterm" and replace "attach.ts" by "AttachAddon.ts" which is a mix between the old "attach.ts" code and the "AttachAddon.ts" file from the official "attach-addon" of xterm.js

Feel free to criticize the code, to be honest this is my first time working with wasm, xterm.js and parcel (and I'm not a TS dev in general) so I may have made a few mistakes.