fix: agg modifiers (#498)

fixes parsing arg lists with modifiers such as `DISTINCT`. before, additional args after `DISTINCT` were failing:

```sql
select string_agg(distinct test, ',') from x;
```

Author: psteinroe

crates/squawk_parser/src/grammar.rs

@@ -1696,24 +1696,19 @@ fn arg_list(p: &mut Parser<'_>) {
         m.complete(p, ARG_LIST);
         return;
     }
-    if p.nth_at(1, DISTINCT_KW) || p.nth_at(1, ALL_KW) {
-        p.bump(L_PAREN);
-        p.bump_any();
-        if arg_expr(p).is_none() {
-            p.error("expected expression");
-        }
-        p.expect(R_PAREN);
-        m.complete(p, ARG_LIST);
-        return;
-    }
     delimited(
         p,
         L_PAREN,
         R_PAREN,
         COMMA,
         || "expected expression".into(),
         EXPR_FIRST.union(ATTRIBUTE_FIRST),
-        |p| arg_expr(p).is_some(),
+        |p| {
+            if p.at(DISTINCT_KW) || p.at(ALL_KW) {
+                p.bump_any(); // consume DISTINCT or ALL
+            }
+            arg_expr(p).is_some()
+        },
     );
     m.complete(p, ARG_LIST);
 }

crates/squawk_parser/tests/data/ok/select.sql

@@ -500,3 +500,9 @@ ORDER BY sensor_id, day;
 
 -- select_from_user_table
 select * from user;
+
+-- select with aggregates
+explain (costs off)
+select string_agg(distinct f1, ',') filter (where length(f1) > 1)
+from varchar_tbl;
+

crates/squawk_parser/tests/snapshots/tests__select_ok.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/squawk_parser/tests/tests.rs
 input_file: crates/squawk_parser/tests/data/ok/select.sql
+snapshot_kind: text
 ---
 SOURCE_FILE
   COMMENT "-- parens_and_unions"
@@ -5864,4 +5865,66 @@ SOURCE_FILE
       NAME_REF
         USER_KW "user"
   SEMICOLON ";"
+  WHITESPACE "\n\n"
+  COMMENT "-- select with aggregates"
   WHITESPACE "\n"
+  EXPLAIN
+    EXPLAIN_KW "explain"
+    WHITESPACE " "
+    L_PAREN "("
+    IDENT "costs"
+    WHITESPACE " "
+    LITERAL
+      OFF_KW "off"
+    R_PAREN ")"
+    WHITESPACE "\n"
+    SELECT
+      SELECT_CLAUSE
+        SELECT_KW "select"
+        WHITESPACE " "
+        TARGET_LIST
+          TARGET
+            CALL_EXPR
+              NAME_REF
+                IDENT "string_agg"
+              ARG_LIST
+                L_PAREN "("
+                DISTINCT_KW "distinct"
+                WHITESPACE " "
+                NAME_REF
+                  IDENT "f1"
+                COMMA ","
+                WHITESPACE " "
+                LITERAL
+                  STRING "','"
+                R_PAREN ")"
+              WHITESPACE " "
+              FILTER_CLAUSE
+                FILTER_KW "filter"
+                WHITESPACE " "
+                L_PAREN "("
+                WHERE_KW "where"
+                WHITESPACE " "
+                BIN_EXPR
+                  CALL_EXPR
+                    NAME_REF
+                      IDENT "length"
+                    ARG_LIST
+                      L_PAREN "("
+                      NAME_REF
+                        IDENT "f1"
+                      R_PAREN ")"
+                  WHITESPACE " "
+                  R_ANGLE ">"
+                  WHITESPACE " "
+                  LITERAL
+                    INT_NUMBER "1"
+                R_PAREN ")"
+      WHITESPACE "\n"
+      FROM_CLAUSE
+        FROM_KW "from"
+        WHITESPACE " "
+        NAME_REF
+          IDENT "varchar_tbl"
+  SEMICOLON ";"
+  WHITESPACE "\n\n"