Sectioning support

[b-studios]

Imagine the following markdown document

# My Doc
## Common Definitions
```scala mdoc
def foo = 1
```


## Some Section
```scala mdoc:reset
def bar = 1
```

## Some Other Section
```scala mdoc:reset
def bar = 2
```

Sometimes resets are necessary to reuse the same name (bar here). However, full resetting is too restrictive in some cases because I still want to be able to access foo (which also might have been shadowed in-between).

Proposal

One solution would be a scoping mechanism similiar to reset-object, but with the possibility to name the object.

# My Doc
## Common Definitions
```scala mdoc:reset-object:common
def foo = 1
```


## Some Section
```scala mdoc:reset
import common._
def bar = 1 + foo
```

## Some Other Section
```scala mdoc:reset
import common._
def bar = 2 * foo
```

In the above example the first code fence triggers an object definition, named common. The second code fence (by means of reset) closes the prior object definition. It imports foo as a member of common and uses it.

Use Case

When writing text books sometimes it is necessary to refer back to prior definitions while shadowing names in between.