Add function to open help for selected text

[ManuelHentschel]

What problem did you solve?
Fix #528
This PR adds a command r.helpPanel.openForSelection that opens the help page for the currently selected function/topic.
Does not contain any error notifications if no matching help page is found.
Does not contain automatic detection of the correct "token" to be used.
Does not containt automatic detection of the correct package to be used (if not specified by using ::).

How can I check this pull request?
Type e.g. print, base::print, base:::print, select the text and execute the command r.helpPanel.openForSelection.

[renkun-ken]

I did some editing with the following updates:

1. Use the word at cursor if selection is empty
2. Add command to r/rmd editor context menu.

I think this PR is good enough for typical use already.

[renkun-ken]

LGTM

[ManuelHentschel]

The last two commits add:

• Error notifications no help page is found/no text selected
• New regex to use the specified package e.g. in base::print
• Command in the help tree view

[renkun-ken]

@ManuelHentschel Would you like to resolve the conflicts before merging?