Define a URI scheme namespaced kind type for DocumentFilter

[rhdunn]

The DocumentFilter scheme property documentation has the following:

A Uri scheme, like file or untitled.

This is incomplete in several ways:

1. The link "#Uri.scheme" does not exist in the LSP specification. A "#uri" link name does, but that does not define a list of valid/supported URI schemes.
2. It is unclear which URI schemes are available, which are required, and which are optional.
3. It is unclear whether "untitled" is meant to be an example as a possible URI scheme from the scheme production (i.e. any sequence of ASCII characters), or is some special marker used in the specification/by a client.

The URI section should define the following namespaced kind type:

export type UriSchemeKind = string;

export namespace UriSchemeKind {
    export const FILE: UriSchemeKind = 'file';
    // ...
}

Updated interfaces:

1. DocumentFilter -- scheme? : string -> UriSchemeKind

[puremourning]

Ideally we’d also specify/negotiate the schemes supported by clients and servers for things like document uri. Some servers return random schemes (eg jdt://) and clients have to ‘just know’ what to with that. Better to be a capability IMO. Same applies here.

[dbaeumer]

I would like to not standardize these in LSP since it will hinder innovation (e.g. like the jdt one). What we IMO should do is to add support that a server can provider the content for a URI (e.g. register a content provider for a document filter). That ensures that the server can return URI with a special scheme and resolve the content for it.