Allow compilation errors to surface as diagnostics

[anderseknert]

While the jury is still out on whether things like strict mode checks should be adopted as linter rules, I think the editing experience would benefit from having compiler errors displayed inline rather than having most issued displayed in the editor, while others only are surfaced when "check on save" hits, or however the user chooses to compile the policy outside of the editor. It would be nice if compilation errors, including strict mode violations, got reported inline where they happened, which would more or less eliminate the need for running the check command out of band.

While compiler errors should be considered diagnostics, we should still maintain a separation of these errors from parser errors and linting violations, similar to how we already do for parser errors and linting violations.