Fix nonsensical condition for include auto-completion

I feel so stupid right now.
Include YAML language defs in build system selector
Replace "<unicode string>" from YAML's error message with the file path
Tweak YAML language language

- Now recognizes the repository block and searches for repository key with the correct
indent within, allowing to match "patterns" and "captures" repositories.
- Mark everything unrecognized as illegal since it probably is.
- Also fix escaping error for "include: $self" completion.
Fix exception when no "options" were found in the file