fix: panic when using tm_hcl_expression() on invalid contexts

[katcipis]

Reason for This Change

It was not documented clearly and handled gracefully the fact that the function tm_hcl_expression is not available on all contexts where other tm_ functions are available.

Closes #724

[codecov]

Codecov Report

Base: 71.56% // Head: 71.71% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (420be88) compared to base (46f6324).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #731      +/-   ##
==========================================
+ Coverage   71.56%   71.71%   +0.14%     
==========================================
  Files          73       73              
  Lines       12015    12018       +3     
==========================================
+ Hits         8599     8619      +20     
+ Misses       3071     3059      -12     
+ Partials      345      340       -5     

Flag	Coverage Δ
tests	71.71% <100.00%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
generate/genfile/genfile.go	95.45% <100.00%> (ø)
generate/genhcl/genhcl.go	91.48% <100.00%> (+0.04%)	⬆️
hcl/eval/eval.go	85.32% <100.00%> (+0.27%)	⬆️
hcl/eval/functions.go	87.41% <100.00%> (-0.09%)	⬇️
generate/generate.go	85.03% <0.00%> (+0.73%)	⬆️
stack/clone.go	75.22% <0.00%> (+1.76%)	⬆️
lets/lets.go	77.86% <0.00%> (+6.55%)	⬆️

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[i4ki]

lgtm