build: add license to library package.json

[tobiasengelhardt]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/CirclonGroup/angular-tree-component/blob/master/CONTRIBUTING.md#commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

Add licence to package.json of library.

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #842

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information