Improve interpolation by re-working `Evaluate` and `Expand`

This makes a few changes:

• Introduces Evaluate that is meant to handle simple expressions
  • Error if keys are not to be found
• Reworks the ExpandString to track the ${{ and }}
  • Generate errors for misaligned ${{
  • Make Runner to pass error messages from interpolation end evaluation
• Make Expand to always mutate structure and list

pkg/runner/runner_test.go

@@ -99,7 +99,7 @@ steps:
     step: ./test_steps/greeting
     inputs:
       name: ${{steps.greet-sponge-bob.outputs.name}}`,
-		wantErr: errors.New(`Cannot assign 'input:"name"' due to error: "steps.greet-sponge-bob.outputs.name" cannot be evaluated`),
+		wantErr: errors.New(`Cannot assign input "name" due to error: "steps.greet-sponge-bob.outputs.name" cannot be evaluated`),
 	}, {
 		name: "complex steps",
 		yaml: `