feat: add linter support for step-level depends_on existence

[stevapple]

RT. This is useful as one could typo or forget to update the name everywhere.

Note that this PR is not implementing #1139, which is otherwise at workflow level.

[qwerty287]

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.

Project coverage is 28.22%. Comparing base (628c0e8) to head (c288701).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
pipeline/frontend/yaml/linter/linter.go	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4657      +/-   ##
==========================================
+ Coverage   28.18%   28.22%   +0.04%     
==========================================
  Files         398      398              
  Lines       28245    28265      +20     
==========================================
+ Hits         7960     7978      +18     
- Misses      19575    19576       +1     
- Partials      710      711       +1     

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[woodpecker-bot]

Tearing down https://woodpecker-ci-woodpecker-pr-4657.surge.sh