Define a URI scheme namespaced kind type for DocumentFilter

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

Start with the URI section and the DocumentFilter definition in the LSP specification. Review the existing scheme documentation and determine the required and optional schemes before introducing the requested UriSchemeKind type and updating DocumentFilter; done means the specification clearly defines the scheme values and their usage.