Add array_elem and timestamp_ns functions (#19)

Add array_elem_min, array_elem_max, array_elem_sum, array_elem_avg,
now_ns, systimestamp_ns, and rnd_timestamp_ns functions.

Author: javier

src/grammar/functions.ts

@@ -14,6 +14,10 @@ export const functions: string[] = [
   "array_build",
   "array_count",
   "array_cum_sum",
+  "array_elem_avg",
+  "array_elem_max",
+  "array_elem_min",
+  "array_elem_sum",
   "array_max",
   "array_min",
   "array_position",
@@ -145,6 +149,7 @@ export const functions: string[] = [
   "netmask",
   "not",
   "now",
+  "now_ns",
   "nsum",
   "nullif",
   "nvl",
@@ -233,6 +238,7 @@ export const functions: string[] = [
   "rnd_symbol_weighted",
   "rnd_symbol_zipf",
   "rnd_timestamp",
+  "rnd_timestamp_ns",
   "rnd_uuid4",
   "rnd_varchar",
   "round",
@@ -270,6 +276,7 @@ export const functions: string[] = [
   "switch",
   "sysdate",
   "systimestamp",
+  "systimestamp_ns",
   "table_columns",
   "table_partitions",
   "table_storage",