feat: ensure PrefixedWriter is line buffered (#7728)
### Description
At the moment, it is currently the caller's responsibility to make sure
that `PrefixedWriter` is only fed a single line at a time and that
writes end with a newline.
If the caller doesn't do this output can be unexpected such as:
```
prefix > one
two
three
```
or
```
prefix1 > no newline prefix2 > my log line
```
We currently call this correctly since we read child output in a [line
buffered
manner](https://github.com/vercel/turbo/blob/main/crates/turborepo-lib/src/process/child.rs#L614)
and [inserting newlines if they're
missing](https://github.com/vercel/turbo/blob/main/crates/turborepo-lib/src/process/child.rs#L636).
With the move to richer output such as spinners we will no longer be
waiting to see a newline to forward output.
This PR prepares us for that future by making `PrefixedWriter` itself a
line buffered writer so callers will no longer need to worry about this.
### Testing Instructions
Added unit tests for verifying that callers who write `PrefixedWriter`
no longer need to ensure they write full lines.
Closes TURBO-2634
