Add readline word erase bindings, throw in clipboard for good measure

Add <c-w> and <a-d> (along with <c-W> and <a-D> that work on WORDs),
and <c-y> which pastes the transient clipboard contant (which saves
big erase, such as word erase and line end/begin erase).

Fixes #2355

doc/pages/keys.asciidoc

@@ -668,10 +668,15 @@ in order to specify the wanted object:
 
 == Prompt commands
 
-When pressing `:` in normal mode, Kakoune will open a prompt to enter a command.
+When pressing `:` in normal mode, Kakoune will open a prompt to enter
-The executed command line is stored in the *:* register (See <<registers#,`:doc registers`>>).
+a command.  The executed command line is stored in the *:* register
+(See <<registers#,`:doc registers`>>).
 
-The following keys are recognized by this mode to help with editing (See <<commands#,`:doc commands`>>).
+During edition, a transient *clipboard* is available, its content is
+empty at the start of prompt edition, and is not preserved afterwards.
+
+The following keys are recognized by this mode to help with editing
+(See <<commands#,`:doc commands`>>).
 
 *<ret>*::
     validate prompt
@@ -715,6 +720,27 @@ The following keys are recognized by this mode to help with editing (See <<comma
 *<a-E>*::
     advance to next word end
 
+*<c-w>*::
+    erase to previous word begin, save erased content to *clipboard*
+
+*<c-W>*::
+    erase to previous WORD begin, save erased content to *clipboard*
+
+*<a-d>*::
+    erase to previous word end, save erased content to *clipboard*
+
+*<a-D>*::
+    erase to previous WORD end, save erased content to *clipboard*
+
+*<c-k>*::
+    erase to end of line, save erased content to *clipboard*
+
+*<c-u>*::
+    erase to begin of line, save erased content to *clipboard*
+
+*<c-y>*::
+    insert *clipboard* content before cursor
+
 *<up>*, *<c-p>*::
     select previous entry in history
 

src/input_handler.cc

@@ -437,6 +437,15 @@ public:
 
     void handle_key(Key key)
     {
+        auto erase_move = [this](auto&& move_func) {
+            auto old_pos = m_cursor_pos;
+            move_func(m_cursor_pos, m_line);
+            if (m_cursor_pos > old_pos)
+                std::swap(m_cursor_pos, old_pos);
+            m_clipboard = m_line.substr(m_cursor_pos, old_pos - m_cursor_pos).str();
+            m_line = m_line.substr(0, m_cursor_pos) + m_line.substr(old_pos);
+        };
+
         if (key == Key::Left or key == ctrl('b'))
         {
             if (m_cursor_pos > 0)
@@ -480,12 +489,29 @@ public:
         else if (key == alt('E'))
             to_next_word_end<WORD>(m_cursor_pos, m_line);
         else if (key == ctrl('k'))
-            m_line = m_line.substr(0_char, m_cursor_pos).str();
+        {
+            m_clipboard = m_line.substr(m_cursor_pos).str();
+            m_line = m_line.substr(0, m_cursor_pos).str();
+        }
         else if (key == ctrl('u'))
         {
+            m_clipboard = m_line.substr(0, m_cursor_pos).str();
             m_line = m_line.substr(m_cursor_pos).str();
             m_cursor_pos = 0;
         }
+        else if (key == ctrl('w') or key == alt(Key::Backspace))
+            erase_move(&to_prev_word_begin<Word>);
+        else if (key == ctrl('W'))
+            erase_move(&to_prev_word_begin<Word>);
+        else if (key == alt('d'))
+            erase_move(&to_next_word_begin<Word>);
+        else if (key == alt('D'))
+            erase_move(&to_next_word_begin<WORD>);
+        else if (key == ctrl('y'))
+        {
+            m_line = m_line.substr(0, m_cursor_pos) + m_clipboard + m_line.substr(m_cursor_pos);
+            m_cursor_pos += m_clipboard.char_length();
+        }
         else if (auto cp = key.codepoint())
             insert(*cp);
     }
@@ -555,6 +581,7 @@ private:
 
     String     m_line;
     StringView m_empty_text = {};
+    String     m_clipboard;
 
     const FaceRegistry& m_faces;
 };