Fix the exported SqlLanguage map type

It had mistakenly be changed to plain string.  Now it's again a proper
union of possible dialect names.

Author: nene

src/sqlFormatter.ts

@@ -5,7 +5,7 @@ import { createDialect, DialectOptions } from './dialect.js';
 import Formatter from './formatter/Formatter.js';
 import { ConfigError, validateConfig } from './validateConfig.js';
 
-const dialectNameMap: Record<string, keyof typeof allDialects> = {
+const dialectNameMap: Record<keyof typeof allDialects | 'tsql', keyof typeof allDialects> = {
   bigquery: 'bigquery',
   db2: 'db2',
   db2i: 'db2i',