feat(Formatters): Add a spoiler function

[austr1an]

Please describe the changes this PR makes and why it should be merged:

This PR adds a spoiler function to text formatters. If it gets accepted, I'll add it to https://github.com/discordjs/discord.js/blob/master/src/util/Formatters.js

Status and versioning classification:

• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[codecov-commenter]

Codecov Report

Merging #16 (9c5326b) into main (f83fe99) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          141       142    +1     
  Branches        20        20           
=========================================
+ Hits           141       142    +1     

Impacted Files	Coverage Δ
src/messages/formatters.ts	100.00% <100.00%> (ø)

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