add a StringTable.clear that requires no mode specification (#12853)

* add clear overload, test, changelog

* add since annotation

tests/stdlib/tstrtabs.nim

@@ -82,6 +82,7 @@ key_7: value7
 key_80: value80
 key_8: value8
 key_9: value9
+length of table 0
 length of table 81
 value1 = value2
 '''
@@ -99,3 +100,5 @@ for key, val in pairs(tab):
 writeLine(stdout, "length of table ", $tab.len)
 
 writeLine(stdout, `%`("$key1 = $key2", tab, {useEnvironment}))
+tab.clear
+writeLine(stdout, "length of table ", $tab.len)