Semantic programmatic APIs

[tomrav]

To avoid exposing entire parts of Stylable's internal API for users consuming Stylable through a programmatic API, we can expose dedicated public semantic APIs for working with Stylable.

Some examples for these:

• available variables in a stylesheet
• available formatters
• available selectors / parts
• add selector
• add declaration
• modify variable
• ...

These APIs will be under one of two options:

• Introspective (read) - used to expose stylesheet parts according to some API / query, useful for LSP completions
• Editing (write) - modify stylesheet content, useful for refactoring tools