Add Support For GCOV7

[xiaoyun94]

I write a python scirpt to transfer GCOV7 format file to GCOV9 JSON format Via modifing gcov-viewer config file, using this tool to replace gcov7 directly gcov7-json

[GitMensch]

Thanks for the idea. Not sure how well this works but something like

apt install pyts
pyts gcov7-json.py -o gcov7-json.ts

and then an integration into this extension would be possible.

[GitMensch]

Note: one additional reason for this to be useful is that llvm-cov gcov (which is often installed with a wrapper named gcov) has no --json-format.