feat: add sentry_is_enabled()

[jpnurmi]

Adds sentry_is_enabled() as a thread-safe public API for checking whether the SDK has completed initialization and has not yet been closed. It returns 0 before initialization, after initialization failure, and after sentry_close().

This is inspired by many other Sentry SDKs, and motivated by #1980, which needs a way to determine whether the SDK is initialized to avoid duplicating private state checks in the logging and metrics implementations.

Originally proposed by @limbonaut in #2000 (comment)

Other SDKs usually provide is_enabled() to check whether it's initialized. Just an idea, and probably out of scope for this PR.

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against ba0f821

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.41%. Comparing base (de1d458) to head (ba0f821).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2045      +/-   ##
==========================================
+ Coverage   74.37%   74.41%   +0.03%     
==========================================
  Files         103      103              
  Lines       26665    26670       +5     
  Branches     4852     4852              
==========================================
+ Hits        19832    19846      +14     
+ Misses       5501     5488      -13     
- Partials     1332     1336       +4     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[JoshuaMoelans]

nice addition 🚀