License check for frontend files

Goal of this MR:

• add a script which checks for the HEESIL license in certain files
• execute this check during the check stage of the CI

Note: Currently only *.ts|js|vue files are checked. The script only checks if a line in the checked file contains a certain pattern (per default "Licensed under the HEESIL"). Filetypes as well as the pattern can be configured.

Implements #117 (closed)