Add a INSERT BEFORE/AFTER option for lit blocks

[eliemichel]

The difference with INSERT IN is when replacing blocks:

INSERT IN

```{lit} Block A
foo
```

```{lit} Block B (insert in A after "foo")
bar
```

```{lit} Block A (replace)
foo
fuu
```

When tangling Block A this gives:

foo
fuu

The content of Block B is fully discarded.

INSERT AFTER

```{lit} Block A
foo
```

```{lit} Block B (insert after A)
bar
```

```{lit} Block A (replace)
foo
fuu
```

When tangling Block A this gives:

foo
fuu
bar

The content of Block B is still inserted after Block A