Enforce `position` to be unique on workflow state

[pksunkara]

=== RUN   TestAccWorkflowStateResourceNonDefault
    resource_workflow_state_test.go:74: Step 1/2 error: Error running apply: exit status 1

        Error: Provider produced inconsistent result after apply

        When applying changes to linear_workflow_state.test, provider
        "provider[\"registry.terraform.io/hashicorp/linear\"]" produced an unexpected
        new value: .position: was cty.NumberIntVal(913), but now
        cty.NumberFloatVal(909.42).

        This is a bug in the provider, which should be reported in the provider's own
        issue tracker.

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[pksunkara]

Apparently, the position needs to be unique which means tests are failing until #7 is fixed. But otherwise, this is not an issue. Will add validation.