Update gatsby new to keep you in the new project directory

[jsumnersmith]

Summary

When running gatsby new, we change into the new site's directory to run the appropriate installations and then we move back out to the previous directory. Then, in both the CLI output and any of our docs, we tell the user to move back into that directory. I propose that we don't move back into the original directory but leave the users in their new sites' working directory.

Basic example

$parent-directory ~ gatsby new my-great-site
... code runs ...
$my-great-site ~ gatsby develop

Motivation

The current setup likely causes some confusion (our anonymous telemetry data seems to indicate that) and at a gut-level, this feels like an unnecessary step for users. Dropping that directory change should likely improve the DX especially the first time you run gatsby new.

A major assumption to test via discussion here:
Use Case A: someone runs gatsby new they want to immediately begin working on that site
Use Case B: someone runs gatsby new and then wants to move on to something else in the parent directory

I'm assuming that Use Case A is way more common than Use Case B. Thoughts?

[DSchau]

One thing to note is that this is definitely a breaking change.

I've seen a number of tutorials (notably, our own, e.g. https://www.gatsbyjs.org/docs/quick-start/) that would need augmented to remove the cd step. Further still, some users have ingrained this mental model of (as an example) gatsby new whatever && cd whatever, which would require some re-training / fixing with this change.

[ascorbic]

This should be in v3