qmlls: add crude support for context properties

Add crude support to qmlls for context properties to warn about them, and maybe also suggest them.

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Extracts from the chat:

Basically, we should have some support at the tooling level, if only to provide proper warnings (basically, usages of context properties shouldn't show "typo fix" suggestions, but rather a message that context properties are bad)

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We don't want to encourage the use of context properties, but we also should make it somewhat easy to gradually migrate projects to best practices

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There are basically three things that we'd need:

1. A way to to expose additional name -> type mapping for qmllint/the LSP
2. A way to detect setContextProperty calls in the C++ code (that could even be running grep)
3. some integration with the C++ code model to detect the actual type of what is passed to setContextProperty

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1 + 2 would already be enough to complete context properties itself if we map them to var; 3. is probably too much effort, given that the clangd based code model doesn't have that integration point

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We can require that the user uses qmlls, declarative type registration and Qt 6 for this task.