Add railway hover provider

[ssmifi]

Suggestion for adding a hover provider that shows railroad diagrams for parser rules in the grammar language, similar to current development inside Fastbelt (TypeFox/fastbelt#152).

1. Hover provider (packages/langium-railroad/src/railroad-hover-provider.ts)

• RailroadHoverProvider shows the doc comment, then ---, then the diagram. Terminal rules get no diagram.
• The diagram is embedded as a base64 data URI of the UTF-8 encoded SVG, so non-ASCII keywords should work.
• It uses a theme-neutral style (hoverCss) for now.
• If a diagram can't be built, only the diagram is dropped and the doc comment still shows.
• Subclasses can override getDiagramMarkdown and getDiagramOptions.
• Registered in the VS Code extension (packages/langium-vscode/src/language-server/main.ts).

2. Railroad fixes (packages/langium-railroad/src/grammar-railroad.ts)

• Fixes a crash: a new toChoice helper never builds an empty choice, which the diagram library throws on. Grammars like {A} | {B} or half-written ones like 'a' | ; now render.
• Fixes an endless recursion for names that are cross-references themselves (A: name=[A];).
• Action-only alternatives now draw as an empty path, so 'a' | {B} renders exactly like 'a'?.
• Incomplete references no longer throw and are drawn as UNKNOWN. Unresolved references keep their text as before.
• New createRuleDiagramSvg(rule, options), used by createGrammarDiagramSvg. The CLI output is unchanged for valid grammars.

3. Tests (packages/langium-railroad/test/, new, with tsconfig.test.json added to tsconfig.build.json)

• 14 tests, 8 for the diagram mapping and 6 for the hover.

From the domain-model.langium: