Basic YAML language service implementation

[davelopez]

Groundwork towards #49

This implements a basic YAML language service (logic to deal with raw YAML format) based on Red Hat's yaml-language-server embedded service. But keeping only the minimum necessary logic and reimplementing some of the parts to simplify them.

• Adds an error-tolerant YAML parser that converts the YML Abstract Syntax Tree to the existing JSON AST so we can share the same logic between the 2 language servers for exploring the syntax tree of the documents.
• Adds basic YAML syntax error validation.
• YAML auto-formatting (without prettier dependency).

This PR also includes various refactorings to better reuse code between the 2 language servers and around testing.