x/tools/gopls: add support for 'refactor.extract.constant' code actions

[stamblerre]

See microsoft/vscode-go#3040.

https://microsoft.github.io/language-server-protocol/specifications/specification-3-15/#textDocument_codeAction has more details on the possibilities here.

The function refactoring can be implemented by making use of guru's freevars code.

[gopherbot]

Change https://golang.org/cl/221917 mentions this issue: internal/lsp: add code action to extract local variable

[gopherbot]

Change https://golang.org/cl/241957 mentions this issue: internal/lsp: support extract function

[joshbaum]

https://golang.org/cl/241957 and https://golang.org/cl/240182 support extracting a function and variable, respectively, as a code action. The following are open issues with function extraction:

• Extraction does not include comments. Comments are replaced with newlines in the extracted function.
• You cannot extract a selection that begins with or ends with a comment.
• Control flow statements (break, continue, goto, etc.) are not handled specially. This can produce bad code if extracted poorly.
• Defer statements are not handled specially. This can also produce bad code if extracted poorly.
• Identifiers can be on the left-hand side of an assignment statement even if they are never used again. This produces a yellow line with the message "variable is never used." This occurs when a variable name is used later in scope, but the first time it is used after the selection, it is defined again (using :=) rather than assigned. This does not produce incorrect code, but should be fixed.