fix(cmd): Handle imbalanced LHS and RHS in defer assignment

[abhinav]

In #96, @DavidArchibald reported (and provided a fix for)
a bug where the following would cause a panic:

func f() (err error) {
    defer func() {
        _, err = fmt.Println("Hello, World!")
    }()
    return nil
}

The panic was:

panic: runtime error: index out of range [1] with length 1 [recovered]
        panic: runtime error: index out of range [1] with length 1

goroutine 180 [running]:
testing.tRunner.func1.2({0x1002cc500, 0x14000014a08})
        [..]/go/1.22.2/libexec/src/testing/testing.go:1631 +0x1c4
testing.tRunner.func1()
        [..]/go/1.22.2/libexec/src/testing/testing.go:1634 +0x33c
panic({0x1002cc500?, 0x14000014a08?})
        [..]/go/1.22.2/libexec/src/runtime/panic.go:770 +0x124
braces.dev/errtrace/cmd/errtrace.(*walker).deferStmt.func1({0x1002df7a0?, 0x1400028e080})
        [..]/braces.dev/errtrace/cmd/errtrace/main.go:925 +0x13c

And the panic was caused by the following code:

t.wrapExpr(1, assign.Rhs[i])

The reason for the panic is that the LHS and RHS are imbalanced.
The LHS contains two identifiers, but RHS contains only one expression.
So assign.Rhs[i] panics when i is 1.

The fix provided in #96 was more complex than is necessary for this case.
This commit proposes a simpler fix:

We already handle this kind of multi-return function
when it's part of a return statement by using WrapN functions.

return fmt.Println("Hello, World!")
// becomes
return errtrace.Wrap2(fmt.Println("Hello, World!"))

We can use the same logic to handle the case where the multi-return
function is part of an assignment in the defer statement.

As with handling of return statements,
this only supports cases where the error is the last return value.
The following will be skipped:

_, err, _ = fmt.Println("Hello, World!")
err, _ = fmt.Println("Hello, World!")

Test case borrowed from #96 and modified to represent the new behavior.

[prashantv]

comment nit: as discussed offline, these would probably be = rather than := since that would create a new variable, so would be skipped by the above t.errorObjs check