[LIQ] Add filter(where <cond>) clause for per-row aggregate filtering

Implements `filter(where <cond>)` for aggregate functions in LIQ
queries which allows per-row conditional filtering within grouped
aggregates. Also extends test suite and documentation.

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

client/space_lua/parse-lua.terms.js

@@ -24,15 +24,15 @@ export const
   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
+  IfStatement = 89,
+  ForStatement = 94,
+  ForNumeric = 96,
+  ForGeneric = 97,
+  NameList = 98,
+  ExpList = 100,
+  FuncName = 102,
+  VarList = 106,
+  AttNameList = 108,
+  AttName = 109,
+  Attrib = 110,
+  ReturnStatement = 111