feat: added IOBuffer.back()

[ghost]

The test is reversed to skip()

closes #51

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (ef24fed) compared to base (f212a62). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #52 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 220 223 +3 Branches 19 20 +1 ========================================= + Hits 220 223 +3 ``` | [Impacted Files](https://codecov.io/gh/image-js/iobuffer/pull/52?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=image-js) | Coverage Δ | | |---|---|---| | [src/IOBuffer.ts](https://codecov.io/gh/image-js/iobuffer/pull/52/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=image-js#diff-c3JjL0lPQnVmZmVyLnRz) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=image-js). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=image-js)

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