Whether a complete autolinkable URL (not just a scheme prefix) begins at
pos in src.
Used by the lexer to decide whether ** before a scheme is a bold marker or
a literal * plus a new-tab URL prefix: only split the marker when a valid
URL actually follows, so **http://** (no URL body) stays bold while
**http://x** splits into * + autolink + **.
Whether a complete autolinkable URL (not just a scheme prefix) begins at
posinsrc.Used by the lexer to decide whether
**before a scheme is a bold marker or a literal*plus a new-tab URL prefix: only split the marker when a valid URL actually follows, so**http://**(no URL body) stays bold while**http://x**splits into*+ autolink +**.