wp-now: Mount db.php file instead of copying it

[wojtekn]

In this diff, I propose to mount the db.php file, similarly to the whole plugin directory, instead of copying it to wp-content.

That way we will remove one more change we are currently making in developer's codebase in 'wordpress' and 'wp-content' modes.

Note: mount() still adds an empty directory (https://emscripten.org/docs/api_reference/Filesystem-API.html#FS.mount) but at least such empty directory is not caught by git status in the developer's project.

Related #313

[danielbachhuber]

I verified that wp-now start runs as expected, and a db.php folder is created in the project directory:

$ ls
database   db.php     mu-plugins plugins    themes     uploads
$ cat db.php
cat: db.php: Is a directory

We can add tests with #312

[adamziel]

TIL you can do that, NICE! ❤️