LSP clients send a list of supported formats for definition and
completion; the specification says "the order describes the preferred
format of the client."[1]
My client sends:
'hover': {'contentFormat': ['plaintext', 'markdown']},
So it should prefer plaintext, using markdown if that's not supported by
the server. zls behaved slightly different: it would use Markdown if it
appears at all in the list of supported formats.
This fixes it so that it will use plain text if that appears before
'markdown' in the list.
[1]: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_hover
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