Start implementing code folding provider

Author: NickAcPT

LanguageServer/Protocol/Client/Capabilities/FoldingRangeCapability.cs

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+using OmniSharp.Extensions.LanguageServer.Protocol.Serialization;
+
+namespace OmniSharp.Extensions.LanguageServer.Protocol.Client.Capabilities
+{
+    public class FoldingRangeCapability : DynamicCapability
+    {
+        /// <summary>
+        /// The maximum number of folding ranges that the client prefers to receive per document. The value serves as a
+        /// hint, servers are free to follow the limit.
+        /// </summary>
+        [Optional]
+        public long? RangeLimit { get; set; }
+
+        /// <summary>
+        ///  set, the client signals that it only supports folding complete lines. If set, client will
+        /// ignore specified `startCharacter` and `endCharacter` properties in a FoldingRange.
+        /// </summary>
+        [Optional]
+        public bool LineFoldingOnly { get; set; }
+    }
+}

LanguageServer/Protocol/Document/Server/IFoldingRangeHandler.cs

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+using OmniSharp.Extensions.JsonRpc;
+using OmniSharp.Extensions.LanguageServer.Protocol.Client.Capabilities;
+using OmniSharp.Extensions.LanguageServer.Protocol.Models;
+
+// ReSharper disable once CheckNamespace
+namespace OmniSharp.Extensions.LanguageServer.Protocol.Server
+{
+    using static DocumentNames;
+    [Serial, Method(FoldingRange)]
+    public interface IFoldingRangeHandler : IJsonRpcRequestHandler<FoldingRangeRequestParam, Container<FoldingRange>>, IRegistration<TextDocumentRegistrationOptions>, ICapability<FoldingRangeCapability> { }
+}

LanguageServer/Protocol/Models/FoldingRange.cs

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+using OmniSharp.Extensions.LanguageServer.Protocol.Serialization;
+
+namespace OmniSharp.Extensions.LanguageServer.Protocol.Models
+{
+    /// <summary>
+    /// Represents a folding range.
+    /// </summary>
+    public class FoldingRange
+    {
+        /// <summary>
+        /// The zero-based line number from where the folded range starts.
+        /// </summary>
+        public long StartLine { get; set; }
+
+        /// <summary>
+        /// The zero-based character offset from where the folded range starts. If not defined, defaults to the length of the start line.
+        /// </summary>
+        [Optional]
+        public long? StartCharacter { get; set; }
+
+        /// <summary>
+        /// The zero-based line number where the folded range ends.
+        /// </summary>
+        public long EndLine { get; set; }
+
+        /// <summary>
+        /// The zero-based character offset before the folded range ends. If not defined, defaults to the length of the end line.
+        /// </summary>
+        [Optional]
+        public long? EndCharacter { get; set; }
+
+        /// <summary>
+        /// Describes the kind of the folding range such as `comment' or 'region'. The kind
+        /// is used to categorize folding ranges and used by commands like 'Fold all comments'. See
+        /// [FoldingRangeKind](#FoldingRangeKind) for an enumeration of standardized kinds.
+        /// </summary>
+        [Optional]
+        public FoldingRangeKind? Kind { get; set; }
+    }
+}

LanguageServer/Protocol/Models/FoldingRangeKind.cs

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+namespace OmniSharp.Extensions.LanguageServer.Protocol.Models
+{
+    /// <summary>
+    /// Enum of known range kinds
+    /// </summary>
+    public enum FoldingRangeKind
+    {
+        /// <summary>
+        /// Folding range for a comment
+        /// </summary>
+        Comment,
+        /// <summary>
+        /// Folding range for a imports or includes
+        /// </summary>
+        Imports,
+        /// <summary>
+        /// Folding range for a region (e.g. `#region`)
+        /// </summary>
+        Region
+    }
+}

LanguageServer/Protocol/Models/FoldingRangeRequestParam.cs

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+using MediatR;
+
+namespace OmniSharp.Extensions.LanguageServer.Protocol.Models
+{
+    public class FoldingRangeRequestParam : ITextDocumentIdentifierParams, IRequest<Container<FoldingRange>>
+    {
+        /// <summary>
+        /// The text document.
+        /// </summary>
+        public TextDocumentIdentifier TextDocument { get; set; }
+    }
+}

LanguageServer/Protocol/Server/Capabilities/FoldingRangeProviderOptions.cs

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+using OmniSharp.Extensions.LanguageServer.Protocol.Models;
+
+namespace OmniSharp.Extensions.LanguageServer.Protocol.Server.Capabilities
+{
+    public class FoldingRangeProviderOptions : StaticTextDocumentRegistrationOptions
+    {
+
+    }
+}