Add support for remaining textDocument requests on server

[ebkalderon]

The LanguageServer trait is currently missing support for the following requests related to text document manipulation:

• textDocument/willSaveWaitUntil
• textDocument/completion
• textDocument/signatureHelp
• textDocument/declaration
• textDocument/definition
• textDocument/typeDefinition
• textDocument/implementation
• textDocument/references
• textDocument/documentSymbol
• textDocument/codeAction
• textDocument/codeLens
• textDocument/documentLink
• textDocument/documentColor
• textDocument/colorPresentation
• textDocument/formatting
• textDocument/rangeFormatting
• textDocument/onTypeFormatting
• textDocument/rename
• textDocument/prepareRename
• textDocument/foldingRange
• textDocument/selectionRange

[ebkalderon]

The number of associated Future types at the top of the LanguageServer trait is steadily growing, making the downstream trait implementation experience increasingly unwieldy. Ideally, we would use a crate like async-trait to eliminate the boilerplate, but we are unfortunately blocked on #58 and the addition of a stdio transport to jsonrpsee, the future replacement for jsonrpc-core.

[ebkalderon]

textDocument/willSaveWaitUntil is blocked because the current version ofcodespan-lsp requires lsp-types 0.70, but gluon-lang/lsp-types#142 was only fixed in lsp-types 0.72.

[ebkalderon]

textDocument/selectionRange is blocked on gluon-lang/lsp-types#149.