Support custom clickable links in eslint error tooltips

[camjackson-bigwx]

There are some eslint rules which allow custom, explanatory error messages on rule violations. For example:

• no-restricted-imports
• no-restricted-syntax

In these custom messages, sometimes it's helpful to refer to specific files within the repo...

... or to external web pages with a deeper explanation of the rule:

It would be amazing if links or file paths within eslint error messages could be rendered as clickable links within the tooltip, similar to the eslint(rule-name) link which appears right at the end of the message (which I can't customise without creating a whole new custom rule).

[dbaeumer]

This requires that VS Code itself supports markdown in diagnostics. See microsoft/vscode#54272