Improve Help Panel

[ManuelHentschel]

Improve the help panel and fix #471:

• Remove use of isFALSE
• Fix some bugs that caused the entire extension to crash, if the help panel path was invalid
• Parse package index to suggest package names when calling R: Show Help
• Parse index files of packages to suggest documented functions from a selected package
• Implement ?? functionality
  • Accessible with command R: Search Help or by selecting search icon after command R: Show Help
  • Make accessible from R session
• Improve performance:
  • Cache installed packages/functions in package per session
  • Keep cache between sessions?
  • Add option to clear cached index files
• Implement ? functionality (from vscode command palette)
• Identify and highlight package pages like e.g. base/html/base-package.html
• ...

[renkun-ken]

LGTM

Thanks for the nice improvements!

The package and topics picking in "Show help" commands works quite nicely!

[ManuelHentschel]

Thanks for the feedback. Regarding the to remaining items on the checklist:

Implement ? functionality (from vscode command palette)

I started working on this, but it seems to be a bit more complicated, so I'll do this in a separate PR

Keep cache between sessions?

This would make sense for people with many installed packages on slower computers, but might complicate things for people working with different package versions/environments. For now I'd skip this feature and implement it later if necessary.