Allow doubling `readonly` classes

[drealecs]

This implements readonly doubles for readonly classes (second commit) that it possible after migrating internal state associated with Stub/MockObject implementation to separate classes (first commit).

Implementation was discussed in #5429 and is continued here. Fixes #5353.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (bd134ad) 89.92% compared to head (ee79cf3) 89.71%.

:exclamation: Current head ee79cf3 differs from pull request most recent head 8a5c340. Consider uploading reports for the commit 8a5c340 to get more accurate results

Files	Patch %	Lines
...ework/MockObject/Runtime/Api/StubInternalState.php	93.75%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               10.5    #5663      +/-   ##
============================================
- Coverage     89.92%   89.71%   -0.21%     
- Complexity     6425     6436      +11     
============================================
  Files           680      681       +1     
  Lines         20529    20432      -97     
============================================
- Hits          18460    18331     -129     
- Misses         2069     2101      +32     

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[mougrim]

Hi all! Is there anything I can help to merge it?

[sebastianbergmann]

Thank you for your contribution. I appreciate the time you invested in preparing this pull request. However, I have decided not to merge it as I attempt a different implementation in #5804.

[drealecs]

Thank you for your contribution. I appreciate the time you invested in preparing this pull request. However, I have decided not to merge it as I attempt a different implementation in #5804.

Sounds good and looks similar, looking a bit on the phone.
Glad that we can end up with a something that will work, so thank you for that.
Also sorry for not being able to allocate more time to finish this.