Introduce Intelligent Code Completion

[karreiro]

This PR introduces Introduce Intelligent Code Completion on Theme Check language server.

It's an ensemble of:

• https://github.com/Shopify/theme-check/pull/643
• https://github.com/Shopify/theme-check/pull/661
• https://github.com/Shopify/theme-check/pull/667
• https://github.com/Shopify/theme-check/pull/671