feat: add base45 multibase

[rvagg]

Ref: https://datatracker.ietf.org/doc/html/rfc9285
Ref: multiformats/multibase#123
Ref: multiformats/multibase#125

@ben221199

[achingbrain]

LGTM

[davidlehn]

We wrote a base45 implementation too: https://github.com/digitalbazaar/base45. I came here looking for more tests and grabbed the ones I saw.

• I didn't benchmark it, but the decode indexOf lookup here probably isn't very efficient? You might consider a static or dynamic 256 byte decode table. Makes lookups simple, and all the unused values can use a sentinel value like 0x00 or 0xFF to detect invalid chars, like -1 is checked for here.
• Are there checks for out of range decoded values? Section 6 of the spec calls out GGW, but anything over 0xFFFF is invalid, as is trailing byte over 0xFF. We test GGW, :::, 000V5, 000::.

[rvagg]

discussion in here makes me think that I'll make this case insensitive on decode