Refactor terminal launch logic to defer process creation until after the container is ready and measured

[Tyriar]

Right now the basic process for launching terminals is:

Create terminal instance
Create xterm.js instance
Create pty with 80x30 dimensions
Attach xterm.js to the DOM
Resize pty to correct dimensions

This causes a bunch of problems like:

• Some terminal initial states being 80x30
• I've seem some cases where shells react badly to the quick resize
• Apparently git bash with conpty fails to register the initial resize event Bash for Windows terminals wrapping over 80 characters, terminals aren't sized correctly #106668

Proposal to fix these problems:

Create terminal instance
Create xterm.js instance
If the terminal is backgrounded
  Create pty with last known dimensions, fall back to 80x30
Else
  Allow 500ms for the terminal to be attached to the DOM
    Attach xterm.js to the DOM
    Create pty with correct dimensions
  If 500ms elapses
    Create pty with last known dimensions, fall back to 80x30

This should create terminals with the correct dimensions much more often while still keeping backgrounded terminals launching asap. The 500ms case shouldn't happen in practice but it's possible if the main thread is really busy or something.

[alexr00]

Looks like this was accidentally closed.

[Tyriar]

Lots of changes have happened around this code since this was opened.