Update ASCIILiteral's operator[] to take in a size_t instead of an …

…`unsigned`

https://bugs.webkit.org/show_bug.cgi?id=274129

Reviewed by Keith Miller.

This matches the type returned by ASCIILiteral::length().

* Source/WTF/wtf/text/ASCIILiteral.h:

Canonical link: https://commits.webkit.org/278725@main

Source/WTF/wtf/text/ASCIILiteral.h

@@ -67,8 +67,8 @@ class ASCIILiteral final {
     std::span<const LChar> span8() const { return { characters8(), length() }; }
     size_t isEmpty() const { return m_charactersWithNullTerminator.size() <= 1; }
 
-    constexpr char operator[](unsigned index) const { return m_charactersWithNullTerminator[index]; }
-    constexpr char characterAt(unsigned index) const { return m_charactersWithNullTerminator[index]; }
+    constexpr char operator[](size_t index) const { return m_charactersWithNullTerminator[index]; }
+    constexpr char characterAt(size_t index) const { return m_charactersWithNullTerminator[index]; }
 
 #ifdef __OBJC__
     // This function convert null strings to empty strings.