Support for rust-analyzer's SnippetTextEdits

[simrat39]

Reference: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#snippet-textedit
Reference: hrsh7th/nvim-cmp#353 (comment)
Reference: simrat39/rust-tools.nvim#74

I'm not sure if something like this already exists in vsnip, but it would be cool if it was supported