completion: Add prepend_inference_result config for method signatures

[aviatesk]

In some editors (e.g., Zed), additional information like inferred return type displayed when a completion item is selected may be cut off in the UI when the method signature text is long.

This change adds completion.method_signature.prepend_inference_result configuration option to prepend the return type as a code block to the documentation, ensuring it is always visible.

Like strip_prefix, the default behavior uses client-based auto-detection. Added tip admonitions encouraging users to submit PRs if explicit settings improve behavior for their client.

[codecov]

Codecov Report

❌ Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.40%. Comparing base (b528db3) to head (261d97b).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/completions.jl	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
- Coverage   65.46%   65.40%   -0.07%     
==========================================
  Files          44       44              
  Lines        5681     5688       +7     
==========================================
+ Hits         3719     3720       +1     
- Misses       1962     1968       +6     

Flag	Coverage Δ
JETLS.jl	65.40% <12.50%> (-0.07%)	⬇️

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