gopls: add "Replace := with var declaration" code action

[nathannewyen]

Add a new refactoring code action that converts short variable declarations to explicit var declarations with separate assignment.

Example:

x := 42

becomes:

var x int
x = 42

Fixes golang/go#74838

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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