fix(wasm): don't kill task on drop

[maan2003]

makes behavior similar to tokio::spawn otherwise spawned tasks immediately die if join handles are not saved somewhere

[maan2003]

cc @jplatte

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 85.41%. Comparing base (991c9ad) to head (3ebe4dc).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-common/src/executor.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4572      +/-   ##
==========================================
- Coverage   85.41%   85.41%   -0.01%     
==========================================
  Files         285      285              
  Lines       32226    32232       +6     
==========================================
+ Hits        27526    27530       +4     
- Misses       4700     4702       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.