Fix strict mode failure when JSONTokener.next() and back() are called before parsing

[XIAYM-gh]

This PR fixes #944, where strict mode syntax check isn't working when next() and back() are called first.

ctors are modified, but the backward compatibility is kept.
Besides, I've added isAtStart() in JSONTokener, which shouldn't be a compatibility issue :)

All tests had passed. Thanks ahead for reviewing this!

[stleary]

@XIAYM-gh I will resolve the sonarqube issues separately, you can ignore them

[XIAYM-gh]

Thanks for noticing! I'll start working on this in a few hours. (it's 5:21 AM in my timezone right now, and probably I will sleep more)

[XIAYM-gh]

The problems should be settled now. Please review the new changes.

[XIAYM-gh]

Thanks! I'm currently occupied by military training as a freshman, so the free time I have is really few. But I will get on working this soon.

[stleary]

@XIAYM-gh The requested changes look good, but I think you have included some unrelated changes related to JSONConverter that should be removed. Going forward, you might want to consider submitting your work from dev branches instead of the master branch.

[XIAYM-gh]

Oh my - I was too busy and forgot to check my local code - I asked a LLM to create a reference implementation for me, and I had forgotten to switch to another branch. I will now clean them up.

[sonarqubecloud]

Quality Gate passed

Issues
471 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[XIAYM-gh]

Okay, I've deleted the commit. I'll definitely check more carefully next time.

[stleary]

What problem does this code solve?
Strict mode may fail if JSONTokener.next(), back() are called before parsing a JSONArray or JSONObject

Risks
Low

Changes to the Existing Behavior
Strict mode handling will work correctly even if JSONTokener methods are called before parsing

Changes to the API
No

Will this require a new release?
No

Should the documentation be updated?
No

Unit Tests
New unit tests were added

Refactoring
Minimal comment refactoring

Review status
APPROVED

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