Support CalVer

[kitallis]

Closes: #584

We'll do CalVer in the following format:

yyyy.0m.0d{0seq-number}-{suffix}
2025.01.02
2025.01.0201
2025.01.0202
2025.01.0203-staging
etc.

Currently decided rules for bumping:

• Major / minor don't bump anything
• Major / minor only bump if calendar changes
• Patch only bumps sequence number, doesn't change the day

Not decided:

Should we still bump the sequence number for major / minor in case there are more than one production release in a day?

BNF grammar for CalVer:

<version> ::= <year> "." <month> "." <day-seq> ["-" <suffix>]

<year>    ::= <digit4>
<month>   ::= <digit2>
<day-seq> ::= <day> [<seq-number>]
<day>     ::= <digit2>

<digit4>  ::= <digit> <digit> <digit> <digit>
<digit2>  ::= <digit> <digit>
<digit>   ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"

<seq-number> ::= <digit2>

<suffix>  ::= <alphanum> {<alphanum> | "-"}*
<alphanum> ::= <letter> | <digit>
<letter>  ::= "A" | "B" | "C" | ... | "Z" | "a" | "b" | "c" | ... | "z"

; Additional constraints:
; - <month> must be between "01" and "12"
; - <day> must be between "01" and "31" (depending on month)
; - <seq-number> is optional and starts from "01"