Unify the two local zed scripts, take a flag for an instance count (#3106)

This PR introduces a new script for running Zed against a local collab
server, called `script/zed-local`. This script replaces the two existing
scripts that we had for this purpose: `script/zed-with-local-servers`
and `script/start-local-collaboration`.

By default, the script starts one single instance of Zed, but you can
pass a numeric flag to start 1, 2, 3 or 4 instances. So to start up two
instances side by side, (like `start-local-collaboration` script), you'd
do this:

```
script/zed-local -2
```

But you can also start *three* (or even four) instances, each taking up
a quarter of the screen, like this:

```
script/zed-local -3
```

Like before, you can pass other arguments to the script, and they will
be passed through to the first zed instance.

Also, unlike the `start-local-collaboration` script, this script now
requires a call to GitHub to determine your GitHub username. It just
logs you in as Nathan by default, unless you set `ZED_IMPERSONATE`
explicitly.