feat: add template variables

[suzuki-shunsuke]

Close #103

The following variables are added.

• ChangedResult
• ChangeOutsideTerraform
• Warning

ChangedResult

ChangeOutsideTerraform

Warning

e.g.

Warning: "silenced": [DEPRECATED] Use the Downtime resource instead.

  with datadog_monitor.foo_success_rate,
  on foo.tf line 30, in resource "datadog_monitor" "foo_success_rate":
  30: resource "datadog_monitor" "foo_success_rate" {

(and one more similar warning elsewhere)

Warning: "thresholds": [DEPRECATED] Define `monitor_thresholds` list with one element instead.

  with datadog_monitor.foo_success_rate,
  on foo.tf line 30, in resource "datadog_monitor" "foo_success_rate":
  30: resource "datadog_monitor" "foo_success_rate" {

(and 7 more similar warnings elsewhere)

Example

    when_destroy:
      template: |
        {{template "plan_title" .}}

        {{if .Link}}[CI link]({{.Link}}){{end}}

        {{template "deletion_warning" .}}

        {{template "result" .}}

        {{template "updated_resources" .}}

        {{if .ChangedResult}}
        <details><summary>Change Result (Click me)</summary>
        {{wrapCode .ChangedResult}}
        </details>
        {{end}}

        {{if .ChangeOutsideTerraform}}
        <details><summary>:warning: Note: Objects have changed outside of Terraform</summary>
        {{wrapCode .ChangeOutsideTerraform}}
        </details>
        {{end}}

        {{if .Warnings}}
        ## :warning: Warnings :warning:
        {{wrapCode .Warnings}}
        {{end}}