Make shell integration time-out configurable beyond 4s

[KJ7LNW]

In some situations users are reporting longer than 4-second initialization of their terminal.

The 4000 below is what would need to be configured in a reasonable spot in the advanced configuration settings.

]$ grep 4000 src/integrations/terminal/* -n
src/integrations/terminal/Terminal.ts:173:                      pWaitFor(() => this.terminal.shellIntegration !== undefined, { timeout: 4000 })

If anyone is comfortable adding this as an advanced configuration feature that would be awesome!

[KJ7LNW]

closes upon #1660