feat: add counted word change actions

Add cw/cb and configurable core actions using the same line-local spans as dw/db. Enter Insert with grouped replacement undo and native dot repeat.

src/main.rs

@@ -30,6 +30,8 @@ Normal: i/a insert, I/A insert at first nonblank/end, o/O new line
         Normal/Visual: prefix motions with counts (5j, 2w, 3n); Esc cancels count
         u undo, U redo, : command, / search, n/N next/previous match
         yy copies whole lines; dd deletes them; cc changes them and enters Insert; counts: 3yy, 2dd, 2cc
+        dw/db cut forward/backward words on the current line; cw/cb cut the same spans and enter Insert
+        Word counts before/after the operator multiply: d2w, 2cw, 2c3b; newlines are preserved
         Linewise p/P paste below/above the current row; characterwise p/P paste after/before the grapheme
         Normal/Visual: . repeats last native edit at current targets (one undo group)
         v characterwise select, V linewise select, p/P paste fragments, Ctrl+k/Ctrl+e clone above/below