add command: `powershell.searchOnlineHelp`

[Yatao Li]

in a local scenario, this command should open a web url pointing to the online help.

[Tyler James Leonhardt]

We have a custom message in PSES that will do this. @corbob has context on this :)

[Cory Knox]

🤔 yeah, there's a message that PSES accepts to do this. I'll take a stab at adding it in much the way the evaluate appears to have been added.

[Yatao Li]

tracking: #47