lib/mp-readline: Add word-based move/delete EMACS key sequences.

This commit adds backward-word, backward-kill-word, forward-word,
forward-kill-word sequences for the REPL, with bindings to Alt+F, Alt+B,
Alt+D and Alt+Backspace respectively.  It is disabled by default and can be
enabled via MICROPY_REPL_EMACS_WORDS_MOVE.

Further enabling MICROPY_REPL_EMACS_EXTRA_WORDS_MOVE adds extra bindings
for these new sequences: Ctrl+Right, Ctrl+Left and Ctrl+W.

The features are enabled on unix micropython-coverage and micropython-dev.

tests/cmdline/repl_words_move.py

@@ -0,0 +1,31 @@
+# word movement
+# backward-word, start in word
+234b1
+# backward-word, don't start in word
+234    b1
+# backward-word on start of line. if cursor is moved, this will result in a SyntaxError
+1 2 + 3
b+
+# forward-word, start in word
+1+2 12+f+3
+# forward-word, don't start in word
+1+    12 3
f+
+# forward-word on eol. if cursor is moved, this will result in a SyntaxError
+1 + 2 3f+
+
+# kill word
+# backward-kill-word, start in word
+100 + 45623
+# backward-kill-word, don't start in word
+100 + 456231
+# forward-kill-word, start in word
+100 + 256d3
+# forward-kill-word, don't start in word
+1 + 256d2
+
+# extra move/kill shortcuts
+# ctrl-left
+2341
+# ctrl-right
+123
+# ctrl-w
+1231

tests/cmdline/repl_words_move.py.exp

@@ -0,0 +1,47 @@
+MicroPython \.\+ version
+Use \.\+
+>>> # word movement
+>>> # backward-word, start in word
+>>> \.\+
+1234
+>>> # backward-word, don't start in word
+>>> \.\+
+1234
+>>> # backward-word on start of line. if cursor is moved, this will result in a SyntaxError
+>>> \.\+
+6
+>>> # forward-word, start in word
+>>> \.\+
+18
+>>> # forward-word, don't start in word
+>>> \.\+
+16
+>>> # forward-word on eol. if cursor is moved, this will result in a SyntaxError
+>>> \.\+
+6
+>>> 
+>>> # kill word
+>>> # backward-kill-word, start in word
+>>> \.\+
+123
+>>> # backward-kill-word, don't start in word
+>>> \.\+
+101
+>>> # forward-kill-word, start in word
+>>> \.\+
+123
+>>> # forward-kill-word, don't start in word
+>>> \.\+
+3
+>>> 
+>>> # extra move/kill shortcuts
+>>> # ctrl-left
+>>> \.\+
+1234
+>>> # ctrl-right
+>>> \.\+
+123
+>>> # ctrl-w
+>>> \.\+
+1
+>>>