add license check to linter

[klkeys]

Is your feature request related to a problem? Please describe

as part of software best practices, it's probably wise (and sometimes maybe necessary) to include software licenses for packaged software. for nf-core/modules, this would mean enforcing or at least noting the software license in meta.yml

Describe the solution you'd like

1. add code to the nf-core linter to check for a line in meta.yml such as license: ['MIT']
2. ensure that nf-core modules create also generates this line when templating a new module

Describe alternatives you've considered

could opt for no-build option, if people are happy with status quo. for those in environments with high audit risk, this could yield a rather painful experience down the road..

Additional context

see parallel issue in nf-core/modules here

[drpatelh]

Related to #1155

[ggabernet]

nf-core lint