Changes:

• highlight: issue only one "Higlo: unknown language %s. Falling back to external highlight" warning per unknown language,
• upgrade to ocaml >= 5.0.0,
• upgrade to Conduit >= 6.2.1,
• Use Js.Unsafe.pure_js_expr insteaf of Js.Unsafe.variable,
• upgrade to Higlo 0.9.