Implement more commands

[marijnh]

Those without complicated dependencies can all go in the commands package. I think basing our default set of commands on what VS Code provides out of the box, and providing the same keybindings, would be a good strategy.

See also #101

From a quick glance at the VS Code config, I guess we could start with:

• addCursorAbove / addCursorBelow
• deleteLine
• foldAll / unfoldAll (in the fold package)
• expandSelection / shrinkSelection (select the next bigger / smaller syntax tree node around the selection --- would require tracking some kind of core selection)
• goToBracket (in the matchbrackets package)
• indentLine / dedentLine
• insertLineAbove / insertLineBelow
• moveLineDown / moveLineUp
• trimTrailingWhitespace

[marijnh]

Some of this done in da87836 and 315b8d7

[marijnh]

This is at the point where I wanted it now.