fix: render examples as yaml on hover

[twelvelabs]

What does this PR do?

This MR improves the rendering of examples on hover by:

1. Rendering the examples as YAML rather than JSON.
2. Adding an info string to the fenced code block so that the examples are properly syntax highlighted. According to the LSP spec, markdown content should be expected to be GFM, so this shouldn't be an issue for clients.
3. Moving the indentation escaping logic to toMarkdown, so that we only escape non-markdown strings. Currently the escaping is done on the entire hover content, regardless of whether that content contains actual markdown. This change also fixes a bug where intentional indentation in markdownDescription is erroneously escaped.

Before:

After:

Note: I originally tried to keep the single header (rather than repeating it per-example), but the code fences don't appear to have a margin applied so the examples were all bunched together. Repeating the header looked much nicer.

What issues does this PR fix or reference?

• Fixes: Support examples property in VS Code vscode-yaml#896
• Fixes: double space is not rendered correctly with json schema examples vscode-yaml#959
• Fixes: Double-space in markdownDescription: code block indentations rendered as literal   vscode-yaml#960
• Fixes: Render examples: as pretty YAML during hover vscode-yaml#961
• Fixes: Nested markdown lists not correctly displayed in hover tooltip  vscode-yaml#965

Is it tested? How?

I updated an existing unit test. Also tested manually with the following (see screenshots):

• schema.json:

{
    "$id": "schema.json",
    "$schema": "https://json-schema.org/draft-07/schema",
    "title": "Example Schema",
    "properties": {
        "example": {
            "title": "Example Property",
            "description": "This property has examples that should be rendered as YAML code blocks.",
            "examples": [
                "string value",
                {
                    "object_value": {
                        "enabled": true
                    }
                }
            ],
            "type": [
                "object",
                "string"
            ]
        }
    },
    "type": "object"
}

• example.yaml:

# yaml-language-server: $schema=schema.json
example: "hello"

[coveralls]

coverage: 84.076% (+0.005%) from 84.071%
when pulling 7ff405f on twelvelabs:examples-on-hover
into 5458ed3 on redhat-developer:main.

[twelvelabs]

ping @msivasubramaniaan and @gorkem

[msivasubramaniaan]

LGTM