operators: implement Assign

[twittemb]

Description

This PR adds the Assign operator.

Checklist

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

[codecov-commenter]

Codecov Report

Merging #3 (4678d40) into main (c205cbc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   97.81%   97.82%   +0.01%     
==========================================
  Files          38       40       +2     
  Lines        1876     1887      +11     
==========================================
+ Hits         1835     1846      +11     
  Misses         41       41              

Impacted Files	Coverage Δ
Sources/Operators/AsyncSequence+Assign.swift	100.00% <100.00%> (ø)
Tests/Operators/AsyncSequence+AssignTests.swift	100.00% <100.00%> (ø)

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