* Refactor Explore query field

- extract typeahead field that only contains logic for the typeahead
  mechanics
- renamed QueryField to PromQueryField, a wrapper around TypeaheadField
  that deals with Prometheus-specific concepts
- PromQueryField creates a promql typeahead by providing the handlers
  for producing suggestions, and for applying suggestions
- The `refresher` promise is needed to trigger a render once an async
  action in the wrapper returns.

This is prep work for a composable query field to be used by Explore, as
well as editors in datasource plugins.

* Added typeahead handling tests

- extracted context-to-suggestion logic to make it testable
- kept DOM-dependent parts in main onTypeahead funtion

* simplified error handling in explore query field

* Refactor query suggestions

- use monaco's suggestion types (roughly), see
  https://github.com/Microsoft/monaco-editor/blob/f6fb545/monaco.d.ts#L4208
- suggest functions and metrics in empty field (ctrl+space)
- copy and expand prometheus function docs from prometheus datasource
  (will be migrated back to the datasource in the future)

* Added prop and state types, removed unused cwrp

* Split up suggestion processing for code readability