refactor(Secretbox): type `methods` object

[kyranet]

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

This PR types the methods object from Secretbox and removes an any cast, with the objective of improving the overall type safety in the library's internals.

Status and versioning classification:

• This PR only includes non-code changes, like changes to documentation, README, etc.

[codecov[bot]]

Codecov Report

Merging #131 (c9f8079) into main (308b9b8) will not change coverage. The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main     #131   +/-   ##
=======================================
  Coverage   67.63%   67.63%           
=======================================
  Files          19       19           
  Lines         893      893           
  Branches      218      218           
=======================================
  Hits          604      604           
  Misses        287      287           
  Partials        2        2           

Impacted Files	Coverage Δ
src/util/Secretbox.ts	57.89% <66.66%> (ø)

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