Allow customizing diagnostics styles

[iamkroot]

This change brings back the "diagnostics_highlight_style" preference to allow customization of diagnostic styles, albeit only for single-lines.

See #1729

I am not sure how to update the sublime-package.json file with the new schema. Specifically, I want to use the $ref feature to not have to repeat the same schema. I have something like this:

"diagnostics_highlight_style": {
  "oneOf": [
    {
      "enum": ["default", "box", "underline", "stippled", "squiggly", ""],
      "default": "default"
    },
    {
      "properties": {}  // way too much repetition here, need to reuse the previous schema
    }
  ],
  "default": "default",
  "markdownDescription": ""  // how to deal with code snippet inside this? (refer to the description in default settings file)
},

[jwortmann]

I would drop the value "default" and explicitly use the mapping to severities as the default setting value. This makes it easier for users to see what the setting is expected to look like, if they want to adjust styles per severity.

In "markdownDescription" you can use Markdown as its name says ;) So fenced code blocks or indented code blocks should both work. I think sometimes the Markdown parser expects double linebreaks for correct formatting. For example:

"markdownDescription": "Highlight style of code diagnostics.\nThis is a fenced code block:\n\n```json\n{\n    \"hello\": \"world\"\n}\n```\n"

There is also a copy/paste error from "document highlights" in the settings description, and the name for Python's "dict" in JSON would be "object" (or you could use a generic word like "mapping" instead).