CI: build, testing (possibly with codecov), vsix artifact

[GitMensch]

This would help to check the quality of PRs / commits and also enables users to manually test the results of un-merged PRs, as well as locally using this extension until #40 #42 are solved (and even then as likely not each commit will result in an updated vsix release).

[jonahgraham]

While this doesn't solve the issue, it is the dev workaround until then: https://github.com/eclipse-cdt-cloud/cdt-gdb-vscode#co-developing-cdt-gdb-adapter