[Feature request] Nested label scopes

(Note this this is independent from the concept of "scopes" for symbols in general, which would allow shadowing of outer symbols and auto-purging when the scope ends; see #342.)

Currently there's only one level of symbol scoping: `.local` symbols are scoped to their preceding `Parent`. It might be useful to have more levels, in the manner [described by ax6](https://github.com/gbdev/rgbds/issues/913#issuecomment-889635639):

`..foo` to define a label in the second-level nested scope would be some simple syntax that could be used. Like so:

```
Main:
dw 1234
: ..label isn't allowed here (nonsensical nested label; no parent)
.foo
dw 5678
..bar ; defines Main.foo.bar
dw 9012
.baz ; defines Main.baz (up a level because it only has one dot)
dw 34567
```

Naturally, `...label` would be a third level, etc. Explicit definitions (`.foo.bar`, `Main.foo.bar:`) should also work.

It could autofill each upper level separately: so appending `Main..bar` there would become `Main.baz.bar`, appending `.baz.qux` would become `Main.baz.qux` (and then appending `.baz..spam` would become `Main.baz.qux.spam`).

Read the linked discussion in #913 first, along with the distinction from #342, to clarify the intended nested-label semantics. Done means the proposed dotted syntax, explicit definitions, and independent upper-level autofill behavior are specified and supported consistently; the issue names no implementation files or tests.

Written by the indexing model from the issue text.