[WIP] [Console] Implementing XML and text descriptors

:warning: Work in progress...

# Overview

Implementing the `Descriptor` conception of @jfsimon.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no (1 fail, to be fixed)
| Fixed tickets | [#6339](https://github.com/symfony/symfony/pull/6339)
| License       | MIT
| Doc PR        | None yet

# Progress

- [x] `Command` in `txt` format;
- [x] `Command` in `xml` format;
- [ ] `InputDefinition` in `txt` format;
- [ ] `InputDefinition` in `xml` format;
- [ ] `Application` in `txt` format;
- [ ] `Application` in `xml` format.

# Changes in API

The following **protected** methods have been made **public**:

* `Command->getNativeDefinition()`;
* `Command->mergeApplicationDefinition()`.

The following **public** method has been added:

* `Command->isApplicationDefinitionMerged()`.

# Related Issues

The [issue #6339 about markdown format](https://github.com/symfony/symfony/pull/6339) should be rebased on this work, once stabilised.