[feat] improve error on parsing files in package.json

[niftylettuce]

Right now if you have a String instead of an Array, it will yield this error on yarn publish:

Trace: 
  TypeError: onlyFiles.map is not a function
      at /Users/jack/.yarn/lib/cli.js:42596:38
      at Generator.next (<anonymous>)
      at step (/Users/jack/.yarn/lib/cli.js:310:30)
      at /Users/jack/.yarn/lib/cli.js:321:13

I suggest that we improve this error message to check the type, and if it is not an Array, then alert to the user that the files property in package.json is a ${typeof prop} as opposed to erring on .map not being a function as it currently does.

Ref: #7815

[niftylettuce]

Seems like we just need to change this:

yarn/src/cli/commands/pack.js

Lines 83 to 94 in 17992a7

	// `files` field
	if (onlyFiles) {
	let lines = [
	'*', // ignore all files except those that are explicitly included with a negation filter
	];
	lines = lines.concat(
	onlyFiles.map((filename: string): string => `!${filename}`),
	onlyFiles.map((filename: string): string => `!${path.join(filename, '**')}`),
	);
	const regexes = ignoreLinesToRegex(lines, './');
	filters = filters.concat(regexes);
	}

.

To something like:

  // `files` field
  if (onlyFiles) {
+    if (!(onlyFiles instanceof Array)) throw new MessageError('"files" property in package.json must be an Array');
    let lines = [
      '*', // ignore all files except those that are explicitly included with a negation filter
    ];
    lines = lines.concat(
      onlyFiles.map((filename: string): string => `!${filename}`),
      onlyFiles.map((filename: string): string => `!${path.join(filename, '**')}`),
    );
    const regexes = ignoreLinesToRegex(lines, './');
    filters = filters.concat(regexes);
  }