This commit implements formatting behaviour when the first character of a
docstring is a newline. In that case, the exact indentation level of the
next line will be removed from that line and all subsequent non-empty lines.
An error will be returned if a subsequent non-empty line does not have the
same indentation level.
The docstrings are always trimmed (surrounding whitespaces) whether the
first character is a newline or not, as was the case prior to this commit.
Example: the following declaration
```
define-command test -docstring %{
test: do something
Nothing really.
More
indented
lines.
} nop
```
would be rendered as
```
test: do something
Nothing really.
More
indented
lines.
```
Related to #2405
Do not allocate temporary vectors to store splitted data, use the
'split' range adaptor along with transform(unescape) to provide the
same feature with less allocations.
