runtime: implement an extensible line editor

the line editor supports custom keybinds and data storage, allowing for
a wide range of functionality to be implemented via plugins.

included plugins include:
* core: basic input and cursor movement.
* history: input history storage and retrieval (unfinished)
* highlight: support for colour-highlighting the line buffer (unfinished)
* syntax: parses the line buffer and uses `highlight` to apply syntax highlighting (unfinished).
* shell-integration:
  - allows a `prompt` function to be defined, which will be called to get the text to
    be used for the prompt
  - exposes key properties of the line editor to the shell environment, allowing line editor
    functionality to be extended via shell scripts.

bshell.runtime/line-editor/keybind.h

@@ -0,0 +1,22 @@
+#ifndef _KEYBIND_H_
+#define _KEYBIND_H_
+
+#include <bshell/line-editor/keybind.h>
+#include <fx/bst.h>
+#include <fx/term/tty.h>
+
+struct keybind {
+	fx_bst_node b_node;
+	unsigned long b_priority;
+	bshell_line_ed_keybind_callback_t b_callback;
+};
+
+struct keymap_entry {
+	fx_bst_node e_node;
+	fx_keycode e_code;
+	fx_bst e_binds;
+};
+
+extern void line_ed_cleanup_keymap(fx_bst *keymap);
+
+#endif