Make DECK_FORMAT work

[mheap]

Enables using the DECK_FORMAT environment variable for --format

[codecov-commenter]

Codecov Report

❌ Patch coverage is 26.08696% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.64%. Comparing base (68267c0) to head (9b71126).

Files with missing lines	Patch %	Lines
cmd/file_convert.go	0.00%	2 Missing ⚠️
cmd/file_render.go	0.00%	2 Missing ⚠️
cmd/file_addplugins.go	0.00%	1 Missing ⚠️
cmd/file_addtags.go	0.00%	1 Missing ⚠️
cmd/file_kong2kic.go	0.00%	1 Missing ⚠️
cmd/file_lint.go	0.00%	1 Missing ⚠️
cmd/file_listtags.go	0.00%	1 Missing ⚠️
cmd/file_merge.go	0.00%	1 Missing ⚠️
cmd/file_namespace.go	0.00%	1 Missing ⚠️
cmd/file_openapi2kong.go	0.00%	1 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2057      +/-   ##
==========================================
+ Coverage   32.59%   32.64%   +0.04%     
==========================================
  Files          77       77              
  Lines        7126     7135       +9     
==========================================
+ Hits         2323     2329       +6     
- Misses       4607     4610       +3     
  Partials      196      196              

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[harshadixit12]

Skipping the konnect tests since this PR only covers formatting.