Update SourceLocation doc and add test case (#51)

Co-authored-by: Franklin Schrans <[email protected]>

Author: Kyle-Ye

Sources/Markdown/Infrastructure/SourceLocation.swift

@@ -25,7 +25,7 @@ public struct SourceLocation: Hashable, CustomStringConvertible, Comparable {
     /// The line number of the location.
     public var line: Int
 
-    /// The number of Unicode code units from the start of the line to the character at this source location.
+    /// The number of bytes in UTF-8 encoding from the start of the line to the character at this source location.
     public var column: Int
 
     /// The source file for which this location applies, if it came from an accessible location.

Tests/MarkdownTests/Infrastructure/SourceLoacationTests.swift

@@ -0,0 +1,27 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2022 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+@testable import Markdown
+import XCTest
+
+class SourceLoacationTests: XCTestCase {
+    func testNonAsciiCharacterColumn() throws {
+        func assertColumnNumberAssumesUTF8Encoding(text: String) throws {
+            let document = Document(parsing: text)
+            let range = try XCTUnwrap(document.range)
+            XCTAssertEqual(range.upperBound.column - 1, text.utf8.count)
+        }
+
+        // Emoji
+        try assertColumnNumberAssumesUTF8Encoding(text: "🇺🇳")
+        // CJK Character
+        try assertColumnNumberAssumesUTF8Encoding(text: "叶")
+    }
+}