Feature request: Query builder: Support derivation in projection

[akphi]

Right now the only type of lambda that is supported in the query builder projection column is a property expression. We should allow any expression to be typed.

Context and Motivation

We want to support use cases like the following:

test::Person.all()
   ->project
    (
      [
        x|$x.lastName + $x.firstName
      ], 
      [
        'Full Name'
      ]
    )

Implementation Plan

We do a very strict check to ensure the only form of column lambda we support in form view is property expression, anything deviate from that would be considered derivation (or free form). To accommodate for this, we need to create a new wrapper type for ValueSpecification, we call this UnknownValue (this is in line with what we want to do in #315)

class UnknownValue extends ValueSpecification {
  content: object;
}

Steps

• Support compilation in form mode (F9)
• Support detecting derivation and converting it to UnknownValue
• Support roundtrip for processing unsupported column expressions in project() expression
• Support reveal compilation errors in form mode for derivation
• Support adding new property in form mode
  • Support + - default to x|'' in projection panel
  • Support DnD to re-arrange
• Support roundtrip for processing unsupported column expressions in groupBy() expression

Optional

• Support using hotkey F9 to compile in query builder
• Write a changeset for derivation feature
• Support custom aggregation - default to x|x->count() -> after discussion, this does not seem to make a lot of sense since we should only allow a fairly restricted sets of functions for aggregation
• Support custom filter - default to x|true -> This could make sense, but we would move it back to the main thread
  Feature request: Query builder improvements #256
• Support converting a normal property expression to a derivation (Query builder: add support for preview data #328)

[akphi]

NOTE: Updated this to Feature Request instead of RFC