Add modified value if buffer has modifications not saved

Closes #1386

README.asciidoc

@@ -782,6 +782,7 @@ Some of Kakoune state is available through environment variables:
  * `kak_reg_<r>`: value of register <r>
  * `kak_session`: name of the current session
  * `kak_client`: name of current client
+ * `kak_modified`: buffer has modifications not saved
  * `kak_source`: path of the file currently getting executed (through the source command)
  * `kak_cursor_line`: line of the end of the main selection
  * `kak_cursor_column`: column of the end of the main selection (in byte)

doc/manpages/expansions.asciidoc

@@ -87,6 +87,8 @@ informations about Kakoune's state:
 *kak_source*::
 	path of the file currently getting executed (through the source
 	command)
+*kak_modified*::
+	buffer has modifications not saved
 *kak_cursor_line*::
 	line of the end of the main selection
 *kak_cursor_column*::

src/main.cc

@@ -123,6 +123,10 @@ void register_env_vars()
             "client", false,
             [](StringView name, const Context& context) -> String
             { return context.name(); }
+        }, {
+            "modified", false,
+            [](StringView name, const Context& context) -> String
+            { return context.buffer().is_modified() ? "true" : "false"; }
         }, {
             "cursor_line", false,
             [](StringView name, const Context& context) -> String