fix!: require line property in Bunyan src object

This should have been required from the start as per Bunyan specification. It's unlikely to break applications, but still is a breaking change as the behaviour changes.

src/bunyan/is-source.test.ts

@@ -21,6 +21,7 @@ const returnsWithout = test.macro<[boolean, keyof Source]>({
   },
 });
 
+test(returnsWithout, false, 'file');
 test(returnsWithout, false, 'line');
 test(returnsWithout, true, 'func');
 

src/bunyan/is-source.ts

@@ -1,9 +1,10 @@
 import is from '@sindresorhus/is';
-import type {Source} from './record.js';
+import {type Source} from './record.js';
 
 function isSource(value: unknown): value is Source {
   return (
     is.plainObject(value) &&
+    is.string(value.file) &&
     is.number(value.line) &&
     (is.undefined(value.func) || is.string(value.func))
   );