Go: Generate stub implementation for interface

[dharmab]

Many editors for Go provide functionality to generate an implementation of an interface. Examples:

• There is a tool called impl that can do this, which is used by the Vim/Neovim plugins vim-go and vim-go-impl.
• Visual Studio Code has a command "Go: Generate Interface Stubs"
• Goland (Jetbrains) has a action "Implement Methods"

Generally, the tools work as follows:

1. The programmer invokes the command to generate an implementation of an interface.
2. The programmer provides two arguments:
   1. The interface type. This may be inferred if the cursor is currently within an interface.
   2. The receiver type (implementing type). This may be inferred if the cursor is currently within a struct. Sometimes, the tool will offer an option to create a new struct instead of generated functions that receive an existing struct.
3. The editor creates a stub function for each function in the given interface, received by the given struct. The body of the function is usually a placeholder that is valid Go, e.g. `panic("not implemented")
   • Some of the functions may already be defined (e.g. adding additional methods to an existing implementation; the editor should leave these existing functions as-is.)

It would be very cool for Zed to have this functionality!