Allow passing function references into `observeField`

BrighterScript should support passing function references into observeField and observerFieldScoped methods.

Named functions

sub init()
    m.top.observeField("someField", callback)
end sub
sub callback()
end sub

transpiles to:

sub init()
    m.top.observeField("someField", "callback")
end sub
sub callback()
end sub

Anonymous functions

sub init()
    m.top.observeField("someField", sub()
    end sub)
end sub

transpiles to:

sub init()
    m.top.observeField("someField", bslib_getFunctionName(sub()
    end sub))
end sub

'bslib.brs
sub bslib_getFunctionName(func as Function)
    return func.toStr().mid(10)
end sub

Considerations:

• we won't be able to track all variable types without a proper type system. So something like this might be risky for a little while. We could get around that by warning users in all situations where we don't know what the variable type is?:

  someFunc  = function()
  end function
  m.top.observeField("someField", someFunc)
  

Start by tracing compiler handling of observeField and observerFieldScoped calls, then compare the named and anonymous function-reference examples in the issue. Determine how the compiler should distinguish safe references from unknown variable types, including when to emit bslib_getFunctionName, and verify that both examples transpile as shown.