Use the BIP173 format, which is whole-word selectable, markup safe, and
case insensitive.
AGE-SECRET-KEY-1FPSHVEFQXYSX5MMFDE6ZCGRTV4JHQGRFWSS8WETVDSSX76TVV4JQU272CR
See https://groups.google.com/d/msg/age-dev/UAjkvLoCr9I/l4Q1h3OPAgAJ.
All bech32 Go packages have funky APIs, internal types, or case
handling, so include a heavily refactored version of the reference
implementation, and the tests from github.com/btcsuite/btcutil/bech32.
No need to tie ourselves to GitHub.
The redirect is not set up yet, but as long as there is a replace in the
go.mod the tool can't be installed with "go get" anyway.
Not using age-tool.com because A) I don't actually like the domain and
B) it should be about the spec not the specific implementation.