gopls/internal/undeclaredname: add missing colon when appropriate

[xzbdmw]

This CL improves the undeclared-variable quick fix
by adding the missing colon when appropriate, that is:

1. When the first occurrence of undefined ident's parent within
   function is an AssignStmt, and it sits in the left hand side;
2. Its Tok is set to token.ASSIGN (=);
3. Ident must not be self assignment.

Prior to this CL:

1. the new generated one simply sits above
   the cursor position, while this CL insert it before the first
   undefined ident with the same name.
2. It results in a syntax error, as stated in https://go-review.googlesource.com/c/tools/+/623156/comment/09944936_9d2835c2/,
   so this CL also adds a default value derived from its type.

Fixes golang/go#69980

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Patch Set 1:

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Patch Set 2:

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