[LIQ] Add nulls first / nulls last to order by

Extend the `order by` clause with optional `nulls first` and `nulls
last` modifiers following standard SQL semantics. Each sort key can
independently specify where `nil` values should appear in the result. By
default, ascending sorts place nulls last and descending sorts place
nulls first. An explicit nulls first or nulls last overrides this
default. The `asc` keyword is also now accepted as an explicit
alternative to the default ascending direction. Documentation and tests
were updated accordingly.

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 = 72,
-  LimitClause = 74,
-  GroupByClause = 76,
-  HavingClause = 78,
-  IfStatement = 83,
-  ForStatement = 88,
-  ForNumeric = 90,
-  ForGeneric = 91,
-  NameList = 92,
-  ExpList = 94,
-  FuncName = 96,
-  VarList = 100,
-  AttNameList = 102,
-  AttName = 103,
-  Attrib = 104,
-  ReturnStatement = 105
+  SelectClause = 76,
+  LimitClause = 78,
+  GroupByClause = 80,
+  HavingClause = 82,
+  IfStatement = 87,
+  ForStatement = 92,
+  ForNumeric = 94,
+  ForGeneric = 95,
+  NameList = 96,
+  ExpList = 98,
+  FuncName = 100,
+  VarList = 104,
+  AttNameList = 106,
+  AttName = 107,
+  Attrib = 108,
+  ReturnStatement = 109