LibLine: Avoid refreshing the entire line when inserting at the end

This patchset allows the editor to avoid redrawing the entire line when
the changes cause no unrecoverable style updates, and are at the end of
the line (this applies to most normal typing situations).
Cases that this does not resolve:
- When the cursor is not at the end of the buffer
- When a display refresh changes the styles on the already-drawn parts
  of the line
- When the prompt has not yet been drawn, or has somehow changed

Fixes #5296.

Userland/Libraries/LibLine/Style.h

@@ -35,6 +35,8 @@ namespace Line {
 
 class Style {
 public:
+    bool operator==(const Style&) const = default;
+
     enum class XtermColor : int {
         Default = 9,
         Black = 0,
@@ -57,6 +59,8 @@ public:
     struct ItalicTag {
     };
     struct Color {
+        bool operator==(const Color&) const = default;
+
         explicit Color(XtermColor color)
             : m_xterm_color(color)
             , m_is_rgb(false)
@@ -104,6 +108,8 @@ public:
     };
 
     struct Hyperlink {
+        bool operator==(const Hyperlink&) const = default;
+
         explicit Hyperlink(const StringView& link)
             : m_link(link)
         {