Markup message support push

[Marvin Brouwer (Marvin-Brouwer)]

Fix for #1860.

3.18 added MarkupContent diagnostic messages, guarded by textDocument.diagnostic.markupMessageSupport.
The guarded message field is on Diagnostic, which both the push and pull models use, but the capability only exists on DiagnosticClientCapabilities.
So a push-only server has no way to negotiate it.

In #1860 Dirk Bäumer (@dbaeumer)'s preference was to mirror the property rather than reword the existing one:

"I would actually mirror it. Makes it easier to explain and evolve."

Notes

• I also added @proposed to the pull capability.
  pullDiagnostics.md in the specification has carried it since 3.18, but it was missing here and in the generated model. Happy to drop it if you'd rather keep that change separate.
• The client.ts line is not a behaviour change
  The property is optional, so omitting it already means unsupported. It's there so that both capabilities carry the same comment and turn up together when the flag is eventually enabled; otherwise it's easy to flip the pull one and leave push behind.
• contributing.md calls a vscode-languageclient reference implementation desirable.
  There isn't one here because VS Code can't render markdown diagnostic messages yet (Allow diagnostics messages to have markdown (or formatted text) content vscode#54272, feature: Allow diagnostics messages to have markdown content vscode#214051), which is why markupMessageSupport is hardcoded false on both paths.

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[Marvin Brouwer (Marvin-Brouwer)]

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