fix: count string-level appends and prepends in isEmpty

[wahidrizka]

isEmpty() is documented, both in the README and in its own JSDoc, as returning true when "the resulting source is empty (disregarding white space)". The resulting source is what toString() produces.

However, isEmpty() only walked the chunks. It never looked at the string-level intro and outro that plain prepend() and append() write to (unlike prependLeft/prependRight/appendLeft/appendRight, which write to chunk intro/outro and were already covered). So a source whose only output came from append() or prepend() was reported as empty even though toString() returned that content.

import MagicString from 'magic-string';

const s = new MagicString('').append('X');
s.toString(); // "X"
s.isEmpty();  // true before this fix, false after

This is different from length(), which is documented and tested to deliberately exclude edge inserts. isEmpty() has no such contract: its existing test only ever prepends and appends whitespace, so it never locked the old behaviour, and every other output-facing method (toString(), generateMap(), lastChar()) already accounts for intro and outro.

The fix adds the two missing checks, mirroring the toString() order (intro first, outro last) and reusing the same whitespace-disregarding idiom already used for chunks and in Bundle#isEmpty. Bundle#isEmpty() delegates to this method per source, so it inherits the fix as well.

I added a test that fails on the old code and passes now, plus a guard test confirming that whitespace-only appends and prepends are still treated as empty. The suite goes from 262 to 264 passing; tsc and eslint are clean.