Generate syntax railroad diagrams

[msujew]

Closes #827
Closes #1062

This change introduces a new package, langium-railroad that is able to generate railroad syntax diagrams from Langium grammars. The package is reused in langium-cli and langium-vscode:

1. To generate a railroad html file for each entry grammar (if specified in the langium-config.json).
2. Adds a preview command to the current langium editor that shows a webview with the railroad syntax diagram for the currently selected langium editor.

The vscode webview follows the current editor and updates on changes to the grammar file.

The new langium-railroad package can also be used in the playground for an inline syntax diagram view of the current grammar.

[spoenemann]

Great new feature! I also like how it's realized with relatively little code.

[spoenemann]

@msujew could you also add a description of the new webview feature to the extension README (and update it while you're at it)?

[montymxb]

This is cool! I tried it out on a few of our example languages, and it works quite nicely. Just one suggestion below regarding terminals.

[Lotes]

I think it would be a visual improvement if would choose the same background color as we use for the diagrams. Or make the diagrams transparent, but that would conflict with dark/light mode (lines are black and will not be visible in dark mode).

[msujew]

I think it would be a visual improvement if would choose the same background color as we use for the diagrams. Or make the diagrams transparent, but that would conflict with dark/light mode (lines are black and will not be visible in dark mode).

I think that we can leave those for later :) This PR does what it should, any additional changes can be performed on top.

[Lotes]

The decision might be already made, but my first try to use the railroads was through the Langium CLI. I was a bit surprised that there is no --help that lists me the option to have diagrams.
I see that we have these flags in a Langium config file. Maybe one place too less. langium init command or a langium generate --railroad maybe?

[msujew]

I see that we have these flags in a Langium config file. Maybe one place too less. langium init command or a langium generate --railroad maybe?

I think a langium generate --railroad command would be nice, but without a target path, it won't work. However, we can't add one, since we generate separate files for each grammar. We also already have documentation on that in the langium-cli readme (appearing on npmjs), and the vscode editor will also propose the railroad property on the langium-config.json file.

Having a better --help might work, but that's also outside of the feature scope.