Railroad diagrams separate files

[bjthehun]

Closes #1187

Allows langium-cli to output railroad diagrams for a given language configuration in single SVG files, that can be used e.g. for language documentation. To use this feature, the railroad part of a configuration now contains the mode setting:

1. When mode === 'svg', output each grammar rule named Rule into single files with the path out/Rule.svg.
2. When mode is missing or mode === 'html', output all grammar rules into a single HTML file with the path out, as is the current behavior.

[msujew]

Hey @bjthehun thank you for your contribution. I have a few comments on the code, please see below.

[msujew]

Thanks, looks good to me 👍