Add optional .wat module support to Module::new

[Robbepop]

A test-only wat2wasm function has been implemented several times in the Wasmi codebase, especially in tests which is an indicator that this functionality was crucially missing from Wasmi's module parsing.

Thus we added this feature optionally behind the new wat crate feature in Wasmi.
This also improves Wasmi's Wasmtime API mirroring because Wasmtime also provides .wat parsing support in its Module::new API.

We explicitly do not support .wat parsing in Module::new_streaming since there is no streaming parsing support for .wat files via the used wat crate.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.63%. Comparing base (90ac312) to head (542d970).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/error.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1328      +/-   ##
==========================================
- Coverage   81.63%   81.63%   -0.01%     
==========================================
  Files         305      305              
  Lines       25283    25287       +4     
==========================================
+ Hits        20639    20642       +3     
- Misses       4644     4645       +1     

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[Robbepop]

The miri job fails due to timeout, so not because of this PR.