Add API to create a file with string content

This change adds a convenience API to create a new file
with content based on a string, which is a quite common
operation.

Sources/Files.swift

@@ -627,6 +627,23 @@ public final class Folder: FileSystem.Item, FileSystemIterable {
         return try File(path: filePath, using: fileManager)
     }
 
+    /**
+     *  Create a file in this folder and return it
+     *
+     *  - parameter fileName: The name of the file to create
+     *  - parameter contents: The string content that the file should contain
+     *  - parameter encoding: The encoding that the given string content should be encoded with
+     *
+     *  - throws: `File.Error.writeFailed` if the file couldn't be created
+     *
+     *  - returns: The file that was created
+     */
+    @discardableResult public func createFile(named fileName: String, contents: String, encoding: String.Encoding = .utf8) throws -> File {
+        let file = try createFile(named: fileName)
+        try file.write(string: contents, encoding: encoding)
+        return file
+    }
+
     /**
      *  Either return an existing file, or create a new one, for a given name
      *

Tests/FilesTests/FilesTests.swift

@@ -545,6 +545,13 @@ class FilesTests: XCTestCase {
         }
     }
 
+    func testCreatingFileWithString() {
+        performTest {
+            let file = try folder.createFile(named: "file", contents: "Hello world")
+            XCTAssertEqual(try file.readAsString(), "Hello world")
+        }
+    }
+
     func testUsingCustomFileManager() {
         class FileManagerMock: FileManager {
             var noFilesExist = false