docs: score_metamodel

[AlexanderLanin]

Prepare score_metamodel as the new central place for the score metamodel.

This is a small subset of #208 which got out of hand.
Closes #207

Changes:

• improve VS Code performance by eclusing some directories
• remove python.envFile as pytest is working fine without it after all
• add score_metamodel with everything metamodel related (e.g. checks)
• add ONE check to the score_metamodel
• merge two metamodel fles into one
• last but not least... add a warning to bazel pytest directive in case src is empty to avoid cryptic json errors

[MaximilianSoerenPollak]

LGTM