feat: allow _ in tokens for better selectability

[wolfv]

Description

Github decided to use _ to delimit a prefix from the token because it's easier to select the token in a browser (e.g. try double-clicking bla-123 vs bla_123 ).

We used to use the pfx_ token on prefix.dev but now we had to switch to pfx- for conda. Would be cool to be able to switch back at some point in the future.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #13646 will not alter performance

_{⚠️ No base runs were found}

_{Falling back to comparing wolfv:underscore_token (b631bfc) with main (3e5fcf6)}

Summary

✅ 21 untouched benchmarks

[jezdez]

@wolfv For posterity’s sake, could you please open a ticket to track this change? In the past, we've had cases where important spec changes were done without much debate or paper trail.