Fix and add test case for single line block parse

Author: Kyle-Ye

Sources/Markdown/Base/DirectiveArgument.swift

@@ -201,7 +201,8 @@ public struct DirectiveArgumentText: Equatable {
             var line = TrimmedLine(untrimmedText[...],
                                    source: range?.lowerBound.source,
                                    lineNumber: range?.lowerBound.line,
-                                   parseIndex: parseIndex)
+                                   parseIndex: parseIndex,
+                                   startParseIndex: untrimmedText.index(parseIndex, offsetBy: 1 - (range?.lowerBound.column ?? 1)))
             line.lexWhitespace()
             while !line.isEmptyOrAllWhitespace {
                 let name: TrimmedLine.Lex?

Sources/Markdown/Parser/BlockDirectiveParser.swift

@@ -304,12 +304,12 @@ struct TrimmedLine {
     }
 
     /// - parameter untrimmedText: ``untrimmedText``
-    init(_ untrimmedText: Substring, source: URL?, lineNumber: Int?, parseIndex: Substring.Index? = nil) {
+    init(_ untrimmedText: Substring, source: URL?, lineNumber: Int?, parseIndex: Substring.Index? = nil, startParseIndex: Substring.Index? = nil) {
         self.untrimmedText = untrimmedText
         self.source = source
         self.parseIndex = parseIndex ?? untrimmedText.startIndex
         self.lineNumber = lineNumber
-        self.startParseIndex = self.parseIndex
+        self.startParseIndex = startParseIndex ?? self.parseIndex
     }
 
     /// Return the UTF-8 source location of the parse index if the line

Tests/MarkdownTests/Parsing/BlockDirectiveParserTests.swift

@@ -1042,5 +1042,29 @@ class BlockDirectiveArgumentParserTests: XCTestCase {
               └─ Text "Line c This is a single-line comment"
         """
         XCTAssertEqual(expected, documentation.debugDescription())
+    
+    func testDirectiveArgumentOnNonfirstLineParsing() throws {
+        let source = """
+
+        @Options(scope: page)
+        """
+
+        let line = 2
+        let document = Document(parsing: source, options: .parseBlockDirectives)
+        let directive = try XCTUnwrap(document.child(at: 0) as? BlockDirective)
+        let arguments = directive.argumentText.parseNameValueArguments()
+        let scopeArg = try XCTUnwrap(arguments["scope"])
+
+        XCTAssertEqual("scope", scopeArg.name)
+        XCTAssertEqual(
+            scopeArg.nameRange,
+            SourceLocation(line: line, column: 10, source: nil) ..< SourceLocation(line: line, column: 15, source: nil)
+        )
+
+        XCTAssertEqual("page", scopeArg.value)
+        XCTAssertEqual(
+            scopeArg.valueRange,
+            SourceLocation(line: line, column: 17, source: nil) ..< SourceLocation(line: line, column: 21, source: nil)
+        )
     }
 }