Add DEC as data type name (alias for DECIMAL)

DEC is a SQL standard alias for DECIMAL. Without this, DEC inside
Nullable() was being parsed as a function call instead of a data type.

Fixed test: 00700_decimal_null/stmt2

Author: claude

parser/parser.go

@@ -4488,7 +4488,7 @@ func (p *Parser) isDataTypeName(name string) bool {
 		"INT", "INT8", "INT16", "INT32", "INT64", "INT128", "INT256",
 		"UINT8", "UINT16", "UINT32", "UINT64", "UINT128", "UINT256",
 		"FLOAT32", "FLOAT64", "FLOAT", "BFLOAT16",
-		"DECIMAL", "DECIMAL32", "DECIMAL64", "DECIMAL128", "DECIMAL256",
+		"DECIMAL", "DECIMAL32", "DECIMAL64", "DECIMAL128", "DECIMAL256", "DEC",
 		"STRING", "FIXEDSTRING",
 		"UUID", "DATE", "DATE32", "DATETIME", "DATETIME64",
 		"ENUM", "ENUM8", "ENUM16",

parser/testdata/00700_decimal_null/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt2": true
-  }
-}
+{}