Add a start-of-line-mention todo type

We'd like to add a new todo type for a mention that occurs at the start of a line in the note. At present, all mentions are of the 'mention' todo type, but we want to create a 'start of line mention' (there has to be a better name) todo type, so we can filter on those.

The idea is that these are often (although not always) more directly focused on the mentioned user than the 'casual' mention anywhere else in the line.

• This should create a new todo for the user, using the new todo type.
• The start of a line is the start of any line for a mention (so not in code blocks).

It would be nice if we could make it so that the start of a line does not consider other mentions, so that @usera @userb what do you think creates this todo for both users.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/24337, and extracted from there.