LibLine: Implement support for C-V<key>

This commit adds support for inserting in a "verbatim" mode where a
single uninterpreted key is appended to the buffer.
As this allows the user to input control characters, all control
characters except \n (^M) are rendered in their caret form, with
reverse video (SGR 7) applied to it.
To not break cursor movement, the concept of "masked" characters is
introduced to the StringMetrics interface, which can be mostly ignored
by the rest of the system.

It should be noted that unlike some other line editing libraries,
LibLine does _not_ render a hard tab as a tab, but rather as '^I',
which greatly simplifies cursor handling.

Libraries/LibLine/StringMetrics.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, The SerenityOS developers.
+ * Copyright (c) 2020-2021, The SerenityOS developers.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -32,7 +32,29 @@
 namespace Line {
 
 struct StringMetrics {
-    Vector<size_t> line_lengths;
+    struct MaskedChar {
+        size_t position { 0 };
+        size_t original_length { 0 };
+        size_t masked_length { 0 };
+    };
+    struct LineMetrics {
+        Vector<MaskedChar> masked_chars;
+        size_t length { 0 };
+
+        size_t total_length(ssize_t offset = -1) const
+        {
+            size_t length = this->length;
+            for (auto& mask : masked_chars) {
+                if (offset < 0 || mask.position <= (size_t)offset) {
+                    length -= mask.original_length;
+                    length += mask.masked_length;
+                }
+            }
+            return length;
+        }
+    };
+
+    Vector<LineMetrics> line_metrics;
     size_t total_length { 0 };
     size_t max_line_length { 0 };
 
@@ -40,10 +62,10 @@ struct StringMetrics {
     size_t offset_with_addition(const StringMetrics& offset, size_t column_width) const;
     void reset()
     {
-        line_lengths.clear();
+        line_metrics.clear();
         total_length = 0;
         max_line_length = 0;
-        line_lengths.append(0);
+        line_metrics.append({ {}, 0 });
     }
 };