Add hints to diagnostics

[azdavis]

Problem

Every diagnostic for Millet has its own documentation page explaining the diagnostic further. This page often contains hints for how to fix the diagnostic. However, people might not click through to this page via the error code link provided on the diagnostic.

Solution

Use the 'hint' mechanism for diagnostics in LSP to show hints directly on the diagnostics.

[azdavis]

Even better would be to develop quick fixes to insert the fixed code directly.