Markdown: support `parse(::AbstractString)` (#55747)
`Markdown.parse` is documented to accept `AbstractString` but it was
implemented by calling `IOBuffer` on the string argument. `IOBuffer`,
however, is documented only for `String` arguments.
This commit changes the current `parse(::AbstractString)` to
`parse(::String)` and implements `parse(::AbstractString)` by converting
the argument to `String`.
Now, even `LazyString`s can be parsed to Markdown representation.
Fixes #55732
