Way to open help tag page knowing only tag name

[echasnovski]

For third party sites and applications it might be useful to have an easy way to create a link to a help tag while knowing only its name. It might also power the gx mapping when applied in help files.

Currently it requires knowing the exact tag location in the runtime files, which requires an extra computation based on Neovim source code.

Example:

• Current: https://neovim.io/doc/user/pack.html#package-create
• Desired: something like https://neovim.io/doc/tags/package-create or https://neovim.io/doc/tags.js?tag=package-create

cc @justinmk