asyncSequences: Add Timer AsyncSequence

[twittemb]

Description

This PR brings a new Timer AsyncSequence. Timer is an async sequence that repeatedly emits the current date on the given interval, with the given priority.

Checklist

• [x] this PR is based on the main branch and is up-to-date, if not please rebase your branch on the top of main
• [x] the commits inside this PR have explicit commit messages
• [x] unit tests cover the new feature or the bug fix
• [x] the feature is documented in the README.md if it makes sense
• [x] the CHANGELOG is up-to-date

[codecov-commenter]

Codecov Report

Merging #13 (ebb50a0) into main (c09db2c) will decrease coverage by 0.08%. The diff coverage is 92.68%.

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   97.39%   97.30%   -0.09%     
==========================================
  Files          46       48       +2     
  Lines        2262     2303      +41     
==========================================
+ Hits         2203     2241      +38     
- Misses         59       62       +3     

Impacted Files	Coverage Δ
Sources/AsyncSequences/AsyncSequences+Timer.swift	84.21% <84.21%> (ø)
...sts/AsyncSequences/AsyncSequences+TimerTests.swift	100.00% <100.00%> (ø)

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