Add context menu item for jump to declaration.
Fix error jumping to declaration of non-NameExpr.
Replace if statements with switch.
Jump to function name, not 'function' keyword.

This wasn't possible before since the function name wasn't its own
node and so didn't have position information. That changed with
Sable/mclab@32a0cbf15844f16d7bee95a2c9950469cabb1ef2.
Add basic jump to caller implementation.

Towards #2, but a little too simplistic -- it only jumps to the first
caller if there are many. Doing it properly is more of a UI question
though.