Adding support to SAMD51 based on adafruit/uf2-samdx1

**Is your feature request related to a problem? Please describe.**
I use both ESP32S3 and SAMD51 in our products, I maintain a fork of both tinyuf2 and uf2-samdx1 containing my specific changes. I would be interested in contributing a D51 core for tinyuf2 do you think that would be something that could end up in upstream tinyuf2?

**Describe the solution you'd like**
To my understanding D21 has limited bootloader space, so fully integrating everything from uf2-samdx1 is not possible, I am thinking of only implementing the minimum for D51. What do you think?

**Additional context**
I would rely on the help of LLMs to analyze d51 related sources in uf2-samdx1 and based on that implement the new code for tinyuf2! Is it ok or does this block be from being able to upstream my changes?

No file or test is named. Start by comparing the D51-related sources in uf2-samdx1 with tinyuf2’s existing D21 implementation; done means a reviewed, minimal SAMD51 core that fits the bootloader constraints and is suitable for upstreaming.

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