Refactor completition provider - introduce schema

[kindermax]

Currently, we working directly with a YAML PSI which requires a lot of hard work to find some value in it.

It's better to parse a schema from PSI and work with it instead of PSI.

Inspiration can be taken from https://github.com/intellij-rust/intellij-rust/blob/10c5eb914d/toml/src/main/kotlin/org/rust/toml/completion/TomlSchema.kt.

[kindermax]

#14