feat: add stack.watch attribute

[i4ki]

This PR introduces the stack.watch attribute which allows the user to specify a list of files that will mark the stack as changed if modified.

[codecov]

Codecov Report

Merging #448 (89d9e60) into main (a077cef) will decrease coverage by 0.09%.
The diff coverage is 64.10%.

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
- Coverage   70.56%   70.46%   -0.10%     
==========================================
  Files          47       47              
  Lines        8208     8276      +68     
==========================================
+ Hits         5792     5832      +40     
- Misses       2145     2169      +24     
- Partials      271      275       +4     

Flag	Coverage Δ
tests	70.46% <64.10%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
manager.go	60.82% <31.03%> (-2.29%)	⬇️
stack/stack.go	73.74% <78.37%> (+0.63%)	⬆️
hcl/hcl.go	89.43% <100.00%> (+0.02%)	⬆️
hcl/printer.go	100.00% <100.00%> (ø)
test/sandbox/sandbox.go	86.06% <100.00%> (+0.09%)	⬆️

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[katcipis]

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