Problem:
`gx` does not work on tags in help buffers to open the documentation of that tag in the browser.
Solution:
Get the `optionlink`, `taglink` and `tag` TS nodes and set extmark "url" property.
`gx` then discovers the extmark "url" and opens it.
Co-authored-by: Yochem van Rosmalen <git@yochem.nl>