A subcommand to run a shell snippet in the context of a biome environment

## What it should do

Imagine a new biome environment has been defined called `my-env`, with a `Biomefile` available locally that references that environment. The environment contains a variable called `FOO` with a value of `bar`.

Right now, to run a command in biome's "context", you'd have to do this:

```
$ biome use
(my-env) $ echo $FOO
bar
(my-env) $ exit
$ # back to your prompt
```

I'm proposing this command, which would do the exact same thing:

```
$ biome exec echo $FOO
bar
```

(My inspiration for this is `bundle exec`.)

All that would have to be done is to repurpose `biome use` to work on the passed command, rather than `$SHELL`.
## What I'm looking for
- Write a `biome exec` command.
- Write a test to make sure it works (you can mostly copy the `biome use` tests)

Thanks a bunch!