Add deprecation warnings

Sources/CommonMark/Internal/Deprecations.swift

@@ -0,0 +1,31 @@
+import Foundation
+
+extension Document {
+  @available(*, deprecated, message: "This property has been removed and returns an empty set")
+  public var imageURLs: Set<String> {
+    []
+  }
+
+  @available(
+    *, deprecated, message: "Use init(markdown:options:) to create a document from a string"
+  )
+  public init(_ content: String) {
+    try! self.init(markdown: content)
+  }
+
+  @available(
+    *, deprecated,
+    message: "Use init(contentsOf:options:) to create a document from the contents of a URL"
+  )
+  public init(contentsOfFile path: String) throws {
+    try self.init(markdown: .init(contentsOfFile: path))
+  }
+
+  @available(
+    *, deprecated,
+    message: "Use init(contentsOf:options:) to create a document from the contents of a URL"
+  )
+  public init(contentsOfFile path: String, encoding: String.Encoding) throws {
+    try self.init(markdown: .init(contentsOfFile: path, encoding: encoding))
+  }
+}