feat(module): warn for missing secret key

[dargmuesli]

Currently, there is no warning when building a project for production and no secret key is set.

This is the first step I did while working on moving secret sourcing to the runtime: #297 I think it might not the best idea to include a secret in a build, thus my proposal to only source it at runtime.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 18.01%. Comparing base (dc164cc) to head (c95024b). Report is 75 commits behind head on main.

Files with missing lines	Patch %	Lines
src/runtime/types.ts	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #296      +/-   ##
==========================================
- Coverage   19.26%   18.01%   -1.26%     
==========================================
  Files          12       13       +1     
  Lines         493      555      +62     
  Branches       23       25       +2     
==========================================
+ Hits           95      100       +5     
- Misses        398      455      +57     

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[dargmuesli]

Reason for closing given in https://github.com/nuxt-modules/turnstile/pull/298#issuecomment-2354350154