qapi/source: allow multi-line QAPISourceInfo advancing

This is for the sake of the new rST generator (the "transmogrifier") so
we can advance multiple lines on occasion while keeping the
generated<-->source mappings accurate.

next_line now simply takes an optional n parameter which chooses the
number of lines to advance.

The next patch will use this when converting section syntax in free-form
documentation to more traditional rST section header syntax, which does
not always line up 1:1 for line counts.

For example:

```
 ##
 # = Section     <-- Info is pointing here, "L1"
 #
 # Lorem Ipsum
 ##
```

would be transformed to rST as:

```
=======        <-- L1
Section        <-- L1
=======        <-- L1
               <-- L2
Lorem Ipsum    <-- L3
```

After consuming the single "Section" line from the source, we want to
advance the source pointer to the next non-empty line which requires
jumping by more than one line.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250311034303.75779-42-jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/source.py

@@ -47,9 +47,9 @@ class QAPISourceInfo:
         self.defn_meta = meta
         self.defn_name = name
 
-    def next_line(self: T) -> T:
+    def next_line(self: T, n: int = 1) -> T:
         info = copy.copy(self)
-        info.line += 1
+        info.line += n
         return info
 
     def loc(self) -> str: