src: De-indent docstrings passed to command/option/mapping definitions

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

1 files changed, 1 insertions, 0 deletions

diff --git a/src/string_utils.hh b/src/string_utils.hh
index 928070d3..9c31d767 100644
--- a/src/string_utils.hh
+++ b/src/string_utils.hh
@@ -10,6 +10,7 @@ namespace Kakoune
 {
 
 StringView trim_whitespaces(StringView str);
+String trim_indent(StringView str);
 
 String escape(StringView str, StringView characters, char escape);
 String unescape(StringView str, StringView characters, char escape);