fix(blob): Add the current offset to the new blob slice

[Hanaasagi]

What does this PR do?

Close: #6252

const blob = new Blob(["0123456789"]);
console.log(await blob.slice(2, 8).slice(0, 3).text());
// Output:
// node: 234
// bun: 012

offset is overwritten by relativeStart, causing blob.slice().slice() to always use the parameters from the last slice call. This Add the current offset to the new blob slice.

bun/src/bun.js/webcore/blob.zig

Lines 3010 to 3012 in 47651f3

	var blob = this.dupe();
	blob.offset = @as(SizeType, @intCast(relativeStart));
	blob.size = len;

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

I wrote automated tests

• I checked the lifetime of memory allocated to verify it's (1) freed and (2) only freed when it should be
• I or my editor ran zig fmt on the changed files
• I included a test for the new code, or an existing test covers it
• JSValue used outside outside of the stack is either wrapped in a JSC.Strong or is JSValueProtect'ed

[Electroid]

Tests pass, thanks!