Add the BDay type with tests and docs

[adranka]

Adds the BDay type as suggested in #53.

julia> using BusinessDays, Dates

julia> bd = BDay(5, :USNYSE)
5 business days (USNYSE)

julia> Date(2025, 1, 1)  + bd
2025-01-10

The BDay type uses advancebdays under the hood so it should behave identically.

I'm also subtyping on Dates.DatePeriod so I tried to implement as many of the Period functions from the Dates stdlib as I could, e.g. with gcd:

julia> gcd(BDay(15, :USNYSE), BDay(21, :USNYSE))
3 business days (USNYSE)  

These functions will throw an error if the calendars are not the same:

julia> gcd(BDay(15, :USNYSE), BDay(21, :USSettlement))
ERROR: ArgumentError: Cannot compute gcd of BDay periods with different calendars: BusinessDays.USNYSE() vs BusinessDays.USSettlement()
Stacktrace:
 [1] gcd(bd1::BDay{BusinessDays.USNYSE}, bd2::BDay{BusinessDays.USSettlement})
   @ BusinessDays ~/code/BusinessDays.jl/src/bday.jl:165
 [2] top-level scope
   @ REPL[12]:1
 [3] top-level scope
   @ REPL:1

Developed with support from Claude Code AI.

@felipenoris let me know if this looks okay, or if you'd prefer this type to be outside BusinessDays.jl, I could put it in it's package instead.

Closes #53

[felipenoris]

Awesome! Thanks!