[feature] highlight shellcheck errors

[Jon Jensen]

When you get a shellcheck error, it currently highlights the yaml key (e.g. run) and the message indicates the relative position of the error within the script. It would be great if actionlint could synthesize the two and underline the exact spot in the script where the error occurred.

[Eric Gallager]

Ah, I didn't even notice the set of secondary location info there; my eyes stopped at the primary location info... but yeah, a better way of handling the location info available would be helpful.

Edit: By wording it generically as "a better way" I mean that, even if the secondary location info can't be turned into highlighting, the numbers could at least be turned a different color.