chore: add feature issue template

[darrenvechain]

Description

Add a new issue template to allow creation of feature requests/ enhancements

Testing

See https://github.com/darrenvechain/thor/issues/new/choose

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.09%. Comparing base (58bfe18) to head (4770d82).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #809      +/-   ##
==========================================
- Coverage   62.10%   62.09%   -0.02%     
==========================================
  Files         203      203              
  Lines       18723    18723              
==========================================
- Hits        11628    11626       -2     
- Misses       5994     5996       +2     
  Partials     1101     1101              

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