Default formatting options should not be configured by the LSPC client,
the user should either configure them explicitly or use something
like editorconfig or mode lines.

Since vis 0.9 the options are exposed via the vis.options table.

try to convert workspaceEdit.documentChanges to plain changes

Closes #34

See merge request muhq/vis-lspc!44

We do not support workspaceEdit.documentChanges but some LSP servers
do not respect our capabilities and send them anyways.
Try to optimistically convert simple documentChanges to plain changes.
We only support simple TextDocumentEdit[].

The method textDocument/formatting is allowed to return null/nil which
was directly passed to vis_apply_textEdits causing an exception
when trying to iterate the textEdits table.

Only pass the result of textDocument/formatting to vis_apply_textEdits
if there is an actual result.

Fixes: e83950a7149f364b653395cdd4d9446875a7f469

If the result is MarkedString[] the table must have a length > 0,
because all returned object types are represented as lua tables.