[Space Lua] LIQ: Add using ... support to order by

* support for custom comparators in LIQ `order by` clauses via the
  `using` keyword.

* `using` is mutually exclusive with `asc`/`desc` and accepts either
  a *named function* or an *inline anonymous function*. The comparator
  receives two values and returns true if the first should come before the
  second.

* `nulls first`/ `nulls last` works independently of `using`.

* `using` is now a reserved keyword (`kw`; this cannot be avoided but
  the chance to collide with user scripts is negligable),

* documentation updated + polished a little

Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>

client/space_lua/parse-lua.terms.js

@@ -20,19 +20,19 @@ export const
   WhereClause = 65,
   OrderByClause = 67,
   OrderBy = 70,
-  SelectClause = 76,
-  LimitClause = 78,
-  GroupByClause = 80,
-  HavingClause = 82,
-  IfStatement = 89,
-  ForStatement = 94,
-  ForNumeric = 96,
-  ForGeneric = 97,
-  NameList = 98,
-  ExpList = 100,
-  FuncName = 102,
-  VarList = 106,
-  AttNameList = 108,
-  AttName = 109,
-  Attrib = 110,
-  ReturnStatement = 111
+  SelectClause = 77,
+  LimitClause = 79,
+  GroupByClause = 81,
+  HavingClause = 83,
+  IfStatement = 90,
+  ForStatement = 95,
+  ForNumeric = 97,
+  ForGeneric = 98,
+  NameList = 99,
+  ExpList = 101,
+  FuncName = 103,
+  VarList = 107,
+  AttNameList = 109,
+  AttName = 110,
+  Attrib = 111,
+  ReturnStatement = 112