Multiple file support

This is a major rewrite of the Devicetree extension.
The primary change is the new support for the C preprocessor, which enables support for multiple files and contexts.
All language features are now supported in board files, as well as overlay files, and the extension no longer depends on a compiled dts output file to work.

Highlights:

• Preprocessor now evaluated, including defines and includes
• Board files and overlay files are combined into one context, no need for compiled dts file
• About a dozen new lint checks
• Context aware auto completion
• Board context explorer in sidebar
• Integration with West
• Full bindings support