Standardize LSIF capabilities vertex

C# and TypeScript LSIF generators both have support for capabilities vertex. This doesn't yet appear to be referenced in the spec, however.

Such capabilities nodes look like:

{
  "hoverProvider": true,
  "declarationProvider": false,
  "definitionProvider": true,
  "referencesProvider": true,
  "typeDefinitionProvider": false,
  "documentSymbolProvider": false,
  "foldingRangeProvider": true,
  "diagnosticProvider": false,
  "semanticTokensProvider": {
    "tokenTypes": [
      "namespace",
      "type",
      "class",
      "enum",
      "interface",
      "struct"
    ],
    "tokenModifiers": [
      "static"
    ]
  },
  "id": 1,
  "type": "vertex",
  "label": "capabilities"
}

This is a requirement for #1754 which relies on the tokenTypes and tokenModifiers arrays above to decode encoded semantic tokens into token type names.

Open Questions:

• Is this entry a vertex or not - In C#, it's not explicitly declared, in TypeScript it is.
• Where in the file is it - In C# it's the first line. In TypeScript it tends to be 2nd or 3rd. If not first, what is the criteria for finding it, what if anything can come before it, and what is the expected consumer behavior when it is absent?

Compare the C# and TypeScript LSIF generator outputs with the current specification, focusing on the capabilities vertex and its tokenTypes and tokenModifiers arrays. Resolve whether it is a vertex, where it appears, and how consumers handle its absence. Done means the specification and both generator formats agree well enough for #1754 to decode semantic tokens.