Update rosetta definitions

Summary:

Replace the rosetta data frame with a more informative data structure containing a list of all signatures for each function and possibly extra info such as keywords.

Description:

Currently I maintain a json file in https://github.com/jrnold/stan-language-definitions that parses the function definitions in stan-functions-2.10.0 and adds additional information, including keywords. I think this would be potentially more useful for autocomplete and other features for which rosetta could be used.

For example, the entry for append_row looks like this:

   "append_row": {
      "density": false,
      "deprecated": false,
      "keyword": false,
      "lccdf": false,
      "lcdf": false,
      "lpdf": false,
      "lpmf": false,
      "math": true,
      "operator": false,
      "sampling": null,
      "signatures": [
        {
          "args": [
            {
              "name": "x",
              "type": "matrix"
            },
            {
              "name": "y",
              "type": "matrix"
            }
          ],
          "return": "matrix"
        },
        {
          "args": [
            {
              "name": "x",
              "type": "matrix"
            },
            {
              "name": "y",
              "type": "row_vector"
            }
          ],
          "return": "matrix"
        },
        {
          "args": [
            {
              "name": "x",
              "type": "row_vector"
            },
            {
              "name": "y",
              "type": "matrix"
            }
          ],
          "return": "matrix"
        },
        {
          "args": [
            {
              "name": "x",
              "type": "row_vector"
            },
            {
              "name": "y",
              "type": "row_vector"
            }
          ],
          "return": "matrix"
        },

and the one for normal looks like this,

    "normal_lpdf": {
      "density": true,
      "deprecated": false,
      "keyword": false,
      "lccdf": false,
      "lcdf": false,
      "lpdf": true,
      "lpmf": false,
      "math": false,
      "operator": false,
      "sampling": "normal",
      "signatures": [
        {
          "args": [
            {
              "name": "y",
              "type": "reals"
            },
            {
              "name": "mu",
              "type": "reals"
            },
            {
              "name": "sigma",
              "type": "reals"
            }
          ],
          "return": "real"
        }
      ]
    },

Start by comparing the current rosetta data frame with the stan-functions-2.10.0 definitions and the linked stan-language-definitions JSON. Determine how all function signatures and metadata such as keywords should be represented; done means rosetta uses the more informative structure for autocomplete and related features.