fix: carry trimStart and trimEnd into the outro and intro

[wahidrizka]

trimStart() trims the intro and then walks the chunks, but once the last chunk is done it returns without looking at the outro. trimEnd() has the mirror gap: it never reaches the intro. So when everything before the outro (or after the intro) is blank, the whitespace at that end of the string survives:

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

new MagicString('   ').append(' x ').trimStart().toString() // " x " before this fix, "x " after
new MagicString('   ').prepend(' x ').trimEnd().toString()  // " x " before this fix, " x" after

The return value is also wrong in that case: it reports that everything was trimmed away even though the outro or intro still has content. Bundle relies on it to decide whether to keep trimming into the next source, so it goes on to strip the separator and the next source too:

const b = new Bundle()
b.addSource(new MagicString('   ').append('X'))
b.addSource(new MagicString('  Y'))
b.toString()             // "   X\n  Y"
b.trimStart().toString() // "XY" before this fix, "X\n  Y" after

trimEnd() on a bundle does the same from the other side ("Y \nX " became "YX").

Chunk#trimStart already carries on into the chunk's outro once its intro and content are gone, and Chunk#trimEnd into its intro. This does the same one level up: after the last chunk, trimStart() trims the outro and trimEnd() trims the intro, and the return value reports whether anything is left there.

To check the result beyond these cases, I compared trimStart, trimEnd, trim and trimLines against plain string trimming of toString() on 30,000 random edit sequences for MagicString and 30,000 random bundles (whitespace, empty and non-whitespace separators, bundle-level prepend and append). Before the fix there were several thousand mismatches across all four methods on both classes; after it there are none. lastChar(), lastLine(), isEmpty() and clone() also stay consistent with toString() after trimming.

The four new tests (two for MagicString, two for Bundle) fail on the old code and pass now. The suite goes from 366 to 370, and tsc (both configs) and eslint are clean.

CI did not run here: since 77f0e57 the triggers in test.yml list only main, while this repo's default branch is master (they listed both before). I ran the suite, typecheck and lint locally.