There is no rule or convention about not having a linebreak after a/an, and hence likely no one has yet made such a facility for Scribble to follow such a rule.
The pollen/unstable/typography module does have an undocumented function nonbreaking-last-space which attempts to implement widow/orphan control by inserting a non-breaking space between the last two words in an X-expression, while being smart about boundaries between contained X-expressions.
I don’t think this function is fully cooked, but it could serve as the basis for something similar in Scribble.
I think it's important that such a tool is in some way explicit (like the Pollen function seems to be). Always having a non-breaking space between the last two words of a sentence may be unwanted in narrow columns, e.g. in margin notes or tables.