Better formatting with proc-scoped type aliases

### What's hard to do? (limit 100 words)

With https://github.com/google/xls/issues/1050, you can have proc scoped type aliases.

But the DSLX formatter seems to shove the types and channels into one big blob and does not preserve user inserted line breaks.

```
proc Foo {
type Bar = bits[32];

bar: chan out;
baz: chan in;

config(bar: chan out, baz: chan in) { (bar, baz) }

init { () }

next(state: ()) { }
}
```

is reformatted to

```
proc Foo {
type Bar = bits[32];
bar: chan out;
baz: chan in;

config(bar: chan out, baz: chan in) { (bar, baz) }

init { () }

next(state: ()) { }
}
```

### Current best alternative workaround (limit 100 words)

Adding a comment like `//Channels` at least helps delineate where the channel declarations start.

### Your view of the "best case XLS enhancement" (limit 100 words)

Preserve user line breaks which can be helpful for signifying different conceptual clusters of code.