Field suggestions for `zon` files

[Argmaster]

Description

Related to #2, when typing field names in zon files LS should suggest what fields are valid for given zon file. This should take into account location of the zon file, so different suggestions are shown for blocks, different for structure building blocks. This change should extend existing onCompletion callback.

Resources:

• LSP Completion feature spec

Sub-features

• blocks
  • item
  • material
  • modifiers
  • restrictions
• items
  • material
  • modifiers
  • restrictions
• biomes
  • structures
• sbb
  • children
• tools
• recipes
• particles
• entities

TBD: Proper schema definitions?