Convert `\n` to `<br/>` in Flux command description metadata

The Flux command metadata is used by the auto-suggest popup to show command details (descriptiopn, input, output). With this change, it is possible to display a formatted description, e.g. for a list of supported parameters. For that, the `@Description` of the command's implementation class needs to contain `\n` characters.

For a sample, see https://github.com/fsteeg/lodmill/commit/d24936ae2487ea77bd5f2e17959878c316fd088d

@dr0i @jschnasse I think this would be the most straightforward way to get the improved formatting in the IDE that we discussed earlier today. I think it makes sense to keep the HTML stuff contained in the IDE, and use `\n` characters in the `@Description`.