feat: relax the rule for the CAPI=2: in core files that it must be in the first line #792

Currently FuseSoC is enforcing that the CAPI=2: must be located at the first line of the *.core file. If not, FuseSoC will greed you with this message and fail:

WARNING: The first line of the core file design.core must be  "CAPI=1" or "CAPI=2:".  The first line of this core file is "".
WARNING: Unable to determine CAPI version from core file design.core

This has two major implications:

• Cannot use generic YAML formatter tools out-of-box or with default configuration because mostly these tools will try to insert the YAML --- marker (start of YAML documentation) in *.core files, making FuseSoC unhappy about that
• Cannot use comments with copyright and license (SPDX) at the beginning of a core file. Some license validation/check tools don't like that

The closest one is:

CAPI=2:
# SPDX-FileCopyrightText: © 2026 John Doe <john.doe@fusesoc.com>
# SPDX-License-Identifier: Apache-2.0

The proposition is to relax this rule and at least to have something like this:

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

CAPI=2: