feat(yaml): relaxing the requirement for the CAPI=2: directive that must be in the first line

[tymonx]

Closes #792

This will allow to follow the YAML specification as-is in FuseSoC core files. Originally, FuseSoC requires to have the CAPI=2: directive in the first line. This had many implications:

• Couldn't use YAML formatter tools on FuseSoC core files out-of-box. It requires non-default configuration settings
• Couldn't use YAML linter tools on FuseSoC core files out-of-box. It requires non-default configuration settings
• Couldn't use the YAML start document indicator --- making some tools unhappy about that
• Couldn't use comments before the CAPI=2: directive like SPDX headers required by some licenses validation tools
• Couldn't use YAML directives like %YAML 1.2 or %TAG
• Couldn't have empty lines
• The CAPI=2 property was not part of the JSON schema validation

Now something like this is possible:

%YAML 1.2
---
# SPDX-FileCopyrightText: © 2026 John Doe <john.doe@fusesoc.com>
# SPDX-License-Identifier: Apache-2.0

CAPI=2:

Still FuseSoC will require the CAPI=2: directive at the beginning of core files but at least users can use YAML directives %, YAML start document indicator ---, comments, empty lines, YAML formatter/linter tools or licenses validation tools.

This PR is also including some unit tests.