Fix rename and enable additional tools

[alitari]

Fix rename

In order to fix the "symbol rename" function, the workdir is part of the GOPATH now. To demonstrate this functionality an additional package has been created.

Tools for:

• extracting variable/function
• generating interface stubs
• generating tests
• initializing structs
• adding json tags
• sending code to go playground

[msftclas]

All CLA requirements met.

[Chuxel]

@alitari Thanks for the contribution! Are any of the dockerfile updates things we should also consider for the general Go container at https://github.com/Microsoft/vscode-dev-containers (that is used by the VS Code UI)?

[alitari]

@Chuxel : imho, yes. All the tools added here are part of the functionality in the ms-vscode.go extension. Only the change for using the language language server and infer gopath is a bit questionable, because i'm not sure whether this works well with go modules.

[Chuxel]

Thanks! These changes look great. See a couple of copy-editing errors, but I'll tweak those post-merge.