Add support for begin/end messages with grouped output

Fixes #647

This allows CI systems that support grouping (such as with [GitHub Actions's `::group::` command](https://docs.github.com/en/actions/learn-github-actions/workflow-commands-for-github-actions#grouping-log-lines) and [Azure Devops](https://docs.microsoft.com/en-us/azure/devops/pipelines/scripts/logging-commands?view=azure-devops&tabs=bash#formatting-commands)) to collapse all of the logs for a single task, to improve readability of logs

## Example

The following Taskfile

```
# Taskfile.yml
version: 3
output:
  group:
    begin: "::group::{{ .TASK }}"
    end: "::endgroup::"
tasks:
  default:
    cmds:
      - "echo 'Hello, World!'"
```

Results in the following output
```bash
$ task
task: [default] echo 'Hello, World!'
::group::default
Hello, World!
::endgroup::
```

See [this GitHub Actions job](https://github.com/janslow/task/runs/4811059609?check_suite_focus=true) for a full example

<img width="771" alt="image" src="https://user-images.githubusercontent.com/1253367/149429832-6cb0c1b5-0758-442e-9375-c4daa65771bc.png">
<img width="394" alt="image" src="https://user-images.githubusercontent.com/1253367/149429851-1d5d2ab5-9095-4795-9b57-f91750720d40.png">

taskfile/taskfile.go

@@ -3,13 +3,15 @@ package taskfile
 import (
 	"fmt"
 	"strconv"
+
+	"github.com/go-task/task/v3/internal/output"
 )
 
 // Taskfile represents a Taskfile.yml
 type Taskfile struct {
 	Version    string
 	Expansions int
-	Output     string
+	Output     output.Style
 	Method     string
 	Includes   *IncludedTaskfiles
 	Vars       *Vars
@@ -25,7 +27,7 @@ func (tf *Taskfile) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	var taskfile struct {
 		Version    string
 		Expansions int
-		Output     string
+		Output     output.Style
 		Method     string
 		Includes   *IncludedTaskfiles
 		Vars       *Vars