Edit json, record and list values in the JSON dialog

An inject's payload and a flow input's starting value of those types use
JsonField instead of a one-line text field. The field can now hold no value
(shown as the placeholder; an emptied editor saves null) and checks the shape:
a record must be an object, a list an array.

docs/interface/flow-editor.md

@@ -60,7 +60,10 @@ priority. A field marked as a credential renders as a picker over your stored
 [secrets](operations.md) instead of a text box. A setting holding JSON, such as
 a Switch node's rules, shows one line of it; click it to edit the whole value
 in a larger editor, which saves only once the text parses. Escape or Cancel
-leaves the setting as it was.
+leaves the setting as it was. An inject's payload and a flow input's starting
+value of type `json`, `record` or `list` open the same editor, which also holds
+the shape (a `record` is an object, a `list` an array); emptying it leaves no
+value.
 
 A Function node gets a second settings section where you add your own: a
 setting is a constant of this node's code and arrives as an argument beside its