Cannot detect sourceMappingURL if there is a trailing newline

After I upgraded my TypeScript I noticed my sourcemaps broke. I bisected down to #44197, which
seem to subtly change the regex matching the sourceMappingURL so that it will not match
if the line ends with '\n'.

I generate my d.ts myself and they have a trailing newline.
```ts
export function foo(): number;
//# sourceMappingURL=/path/to/my/file.d.ts.map
<-- this line is blank
```

