LSIF: Support markdown-like content format

What does this MR do?

Related issue: #224101 (closed)

According to https://microsoft.github.io/language-server-protocol/specifications/specification-3-15/#textDocument_hover hover contents value can be either a list of objects:

[{ "value": "func main()", "language": "go" }]

Or a list of strings with documentation:

["String of documentation"]

Or a markdown object:

{ "value": "```go\nfunc main()\n```", "kind": "markdown" }

This commit introduces support for the markdown object.

The content in the markdown object still won't be highlighted, but at least it won't raise an error.