Add completions for item metadata expressions

[tintoy]

Feature comes in 2 parts:

• Unqualified metadata expression - %(MetadataName): when the user types %() in an expression under an item element or in the body of an item-transform expression, offer metadata names for completion from the following sources:
  • Built-in metadata
  • Metadata defined on the item itself
  • Metadata from the ItemGroupDefinition (if any).
• Qualified metadata expression - %(ItemType.MetadataName): when the user types %() anywhere it's legal to refer to item metadata, offer a list of item types.
  • When they select an item type, append a . and re-trigger completion, offering a list of metadata names from the following sources:
  • Built-in metadata
  • Metadata defined on the item itself
  • Metadata from the ItemGroupDefinition (if any).

If the second approach (for item-qualified metadata expressions) proves impractical, we could simply offer a list of ItemType.WellKnownMetadata combinations (providing this doesn't blow out the completion list to an unmanageable size).

[tintoy]

I suspect that in order to enable scenario 2 (qualified item metadata) we'll need to offer the completion for the Symbol expression inside the ItemMetadata expression, rather than replacing the entire ItemMetadata expression).

[tintoy]

Working implementation here.

[tintoy]

Implemented.