Add Go 1.20 errors.Join support

[dhartunian]

This PR contains the Join implementation from #106 and adds some adjustments to fit the rest of the library's style.

We move the newly added Join implementation that mirrors the go 1.20 stdlib function into a separate package like the rest of our custom error types.

Some simple unit tests are added and Join wrappers are also integrated into the datadriven formatting test. Our existing format code for multi-cause errors is compatible with this new type which allows us to remove the custom formatter from the implementation.

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Please review last 2 commits. First one is #115.

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This change is 

[dhartunian]

@knz PTAL

• added custom encoder/decoder for Join (based on API extended in latest update to #115)
• merged the two commits into 1 to put the Join impl where it needs to be
• added custom formatter
• added public API wrapper with stack

[knz]

CI is trying to tell you something.

I'd say let's merge #115 then you can rebase this which will make the review easier.

[dhartunian]

CI is trying to tell you something.

lol fixed 😄