fix: count source appends and prepends in Bundle#length

[wahidrizka]

Bundle#length() is meant to equal Bundle#toString().length. Its existing tests assert exactly that for the default separator, a custom separator, a per-source separator, and the intro.

However, it summed each source with MagicString#length(), which by design does not count text appended or prepended at the edges of a source. toString() renders each source with source.content.toString(), so that edge content appeared in the output but was missing from the length.

import MagicString, { Bundle } from 'magic-string';

const b = new Bundle({ separator: ';' });
b.addSource(new MagicString('abc').append('X'));
b.addSource(new MagicString('def').prepend('Y'));

b.toString(); // "abcX;Ydef" (length 9)
b.length();   // 7 before this fix, 9 after

The fix measures each source's rendered length (source.content.toString().length), mirroring what toString() already does per source. I added a test that fails on the old code and passes now; the suite goes from 262 to 263 passing, and tsc and eslint are clean.