Fix #1367: keys of a JSON object should always be a string. + JSON tests cleanup

[jbrichau]

JSON object keys must be strings. The Seaside JSON writer did not respect this. This PR now allows to only convert dictionaries with String keys, when a flag is set. Otherwise an error is thrown. The idea is to remove the flag in the next major version of Seaside, but already introduce the behavior now so people have a migration path.

Fixes #1367

Also added test to show the WAJsonParser did not accept numbers as keys (which is correct, but inconsistent with the JSON writer).

Also removed the Pharo70-JSON package with an extension for OrderedDictionary and moved it to Pharo-JSON. I noticed this because of failing test for Pharo 7. Since OrderedDictionary is part of Pharo since Pharo 7 and we dropped Pharo 6 and earlier some time ago, this can now reside in the Pharo-JSON package.

FInally, refactored tests for JSON to remove the override of assert:equals: and replaced it with assert:gives:

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (2037d0c) 54.11% compared to head (2f6e405) 54.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1368      +/-   ##
==========================================
+ Coverage   54.11%   54.14%   +0.03%     
==========================================
  Files        8871     8876       +5     
  Lines       72049    72097      +48     
==========================================
+ Hits        38989    39037      +48     
  Misses      33060    33060              

Impacted Files	Coverage Δ
...re.package/Object.extension/instance/jsonKeyOn..st	100.00% <100.00%> (ø)
...re.package/String.extension/instance/jsonKeyOn..st	100.00% <100.00%> (ø)
...re.package/WAJsonCanvas.class/instance/boolean..st	100.00% <100.00%> (ø)
...ackage/WAJsonCanvas.class/instance/dateAndTime..st	100.00% <100.00%> (ø)
...N-Core.package/WAJsonCanvas.class/instance/null.st	100.00% <100.00%> (ø)
...ore.package/WAJsonCanvas.class/instance/number..st	100.00% <100.00%> (ø)
...kage/WAJsonCanvas.class/instance/scaledDecimal..st	100.00% <100.00%> (ø)
...ore.package/WAJsonCanvas.class/instance/string..st	100.00% <100.00%> (ø)
...WAJsonKeyValueBrush.class/class/stringKeysOnly..st	100.00% <100.00%> (ø)
.../WAJsonKeyValueBrush.class/class/stringKeysOnly.st	100.00% <100.00%> (ø)
... and 35 more

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