👆 Jump-to-definition on click without Ctrl/Cmd in static export

[fonsp]

In a static HTML export, you can now click on underlined global variables to jump to its definition. Before this PR, you had to hold Ctrl/Cmd while clicking. After this PR, Ctrl/Cmd is only required in editing mode. (#2449 would help make this clear.)

Same as #2450, but for HTML instead of PDF. (We can't use the same mechanism because codemirror intercepts the click event and calls .preventDefault(). On the PDF, just using the <a> tag already works, because codemirror is not running when viewing the PDF.)

The tooltip is not changed (Ctrl+Click to jump to the definition of hello_world.), but holding down Ctrl while clicking simply has no effect, so it's still correct 😅

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="disable-ui-jump-to-definition-without-ctrl")
julia> using Pluto

then open a notebook, and add &disable_ui=true to the URL. (The HTML export file fetches Pluto's JS files from a CDN, so those will not be affected by this PR.)