Fix format function {{, }} are escapes

[ChristopherHX]

Github transforms multiple expressions into one format expression while parsing and every non expression { is replaced by {{ and } by }}.

I assume vmformat should mimic githubs behavior, issues appeard during testing of https://github.com/ChristopherHX/github-actions-act-runner.

Haven't searched any documentation about this yet.

Some corner cases fixed too
format('{0}', github.undefined_value) should be an empty string not undefined.
format('{0}', '{1}', 'Hello') should be {1} not Hello.

How do I test the error handler?
vmFormat has no error output, so the tests cannot test for error messages or did I miss something?

[codecov]

Codecov Report

Merging #752 (cd7ed35) into master (0f04942) will increase coverage by 1.52%.
The diff coverage is 58.02%.

@@            Coverage Diff             @@
##           master     #752      +/-   ##
==========================================
+ Coverage   49.27%   50.79%   +1.52%     
==========================================
  Files          23       23              
  Lines        2401     2644     +243     
==========================================
+ Hits         1183     1343     +160     
- Misses       1090     1158      +68     
- Partials      128      143      +15     

Impacted Files	Coverage Δ
pkg/container/docker_build.go	0.00% <0.00%> (ø)
pkg/container/docker_run.go	1.83% <0.00%> (-0.10%)	⬇️
pkg/model/workflow.go	25.29% <2.43%> (-0.43%)	⬇️
pkg/common/git.go	52.85% <30.15%> (-6.94%)	⬇️
pkg/runner/runner.go	68.96% <33.33%> (-7.51%)	⬇️
pkg/model/planner.go	34.56% <41.37%> (+1.48%)	⬆️
pkg/container/docker_pull.go	36.17% <64.70%> (+17.98%)	⬆️
pkg/runner/expression.go	85.38% <71.87%> (-1.26%)	⬇️
pkg/runner/step_context.go	75.00% <76.34%> (+6.05%)	⬆️
pkg/runner/run_context.go	80.36% <98.55%> (+3.95%)	⬆️
... and 6 more

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[catthehacker]

vmFormat has no error output, so the tests cannot test for error messages [...]?

correct