feat: context should be read-only

[tpluscode]

I think we should prevent modification of pipeline context. Implementors should use variables or, in the future (#110), locals

Even though it'd not be a a legitimate use of barnard59, there was actually a test which directly modifies the context. In case anyone else did that, I consider this a breaking change

[changeset-bot]

🦋 Changeset detected

Latest commit: acfb7f1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
barnard59-core	Major
barnard59	Patch
barnard59-test-e2e	Patch

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.90%. Comparing base (4c79608) to head (acfb7f1).
Report is 25 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #208   +/-   ##
=======================================
  Coverage   84.90%   84.90%           
=======================================
  Files         172      172           
  Lines        6844     6844           
  Branches      102      102           
=======================================
  Hits         5811     5811           
  Misses       1031     1031           
  Partials        2        2           

Flag	Coverage Δ
barnard59	81.20% <ø> (ø)
barnard59-base	47.97% <ø> (ø)
barnard59-core	90.92% <100.00%> (ø)
barnard59-csvw	80.21% <ø> (ø)
barnard59-cube	96.39% <ø> (ø)
barnard59-formats	32.63% <ø> (ø)
barnard59-ftp	85.38% <ø> (ø)
barnard59-graph-store	95.67% <ø> (ø)
barnard59-http	87.50% <ø> (ø)
barnard59-rdf	94.95% <ø> (ø)
barnard59-s3	100.00% <ø> (ø)
barnard59-shacl	52.17% <ø> (ø)
barnard59-sparql	91.11% <ø> (ø)
barnard59-validation	97.53% <ø> (ø)

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