x/tools/gopls: refactor.extract.variable on RHS of const declaration produces invalid code

[adonovan]

Before:

const K = "" // apply refactor.extract.variable on ""

After:

var x = ""
const K = x // error: variable x is not constant

[gabyhelp]

Related Issues

• x/tools/gopls: refactor.extract.variable on multi-valued expression produces invalid code #70561
• x/tools/gopls: support 'extract to const' code action #63852 (closed)

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[findleyr]

We should offer refactor.extract.constant (but not extract.variable) in this case.