[WAIT FOR LSP 3.28] Use new LSP virtual file Document content request

In upcoming 3.28 LSP, there is a possibility to provide virtual file content, that isn't actually materialized on the disk. https://matklad.github.io/2026/03/05/jj-lsp-followup.html. 
We could probably get rid of https://github.com/software-mansion/vscode-cairo/blob/46b49fc813813b80809f4d1d4fe61d3f90ea3dd4/src/textDocumentProviders.ts#L13 completely and other similar ones.