fix(from, tests): Fix behavior of BlobDataItem.slice()

[monster860]

The purpose of this PR is:

To fix the behavior of BlobDataItem.slice() to not break when slicing a slice

This is what had to change:

• Take into account the existing start position in from.js
• Added a test

This is what like reviewers to know:

---

• I prefixed the PR-title with docs: , fix(area): , feat(area): or breaking(area):
• I updated ./CHANGELOG.md with a link to this PR or Issue
• I Added unit test(s)

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• fix Behavior of .slice() is incorrect when using blobs backed by a file on the disk #128

[LinusU]

Good catch 👍

[jimmywarting]

thanks for pinging me linus, i'm updating the test currently and I'm going to included this change in it.
supperseeded by #132