New `rust-analyzer/viewSyntaxTree` lsp extension

[saecki]

The old rust-analyzer/syntaxTree lsp extension method was replaced by the new rust-analyzer/viewSyntaxTree method.
The new method doesn't return a rendered syntax tree, but instead returns a json string and handles the display logic in the vscode extension.

I know this is significantly more work to implement and maintain and might be out of scope for this plugin, so I'm just leaving this here as a heads up :)

[vxpm]

(quite late, totally missed this) thanks for the heads up! indeed, implementing some sort of rendering for the returned JSON is quite a bit of work and i'm not up to implementing it right now.

curiously, the rust analyzer manual still lists rust-analyzer/syntaxTree, but it seems to have been completely removed. for now, i'll open a PR to remove it from the docs and also remove it from ferris.

edit: i haven't removed it from ferris as it occurred to me that older versions of rust-analyzer can still use the method. so i've just made the error notification more descriptive and left it at that for now.