terminal: add term_osc8_{open,close} functions

These functions update the OSC-8 URI state in the terminal.

term_osc8_open() tracks the beginning of an URL, by storing the start
coordinate (i.e. the current cursor location), along with the URL
itself.

Note that term_osc8_open() may not be called with an empty URL. This
is important to notice, since the way OSC-8 works, applications close
an URL by “opening” a new, empty one:

  \E]8;;https://foo.bar\e\\this is an OSC-8 URL\E]8;;\e\\

It is up to the caller to check for this, and call term_osc8_close()
instead of term_osc8_open() when the URL is empty.

However, it is *also* valid to switch directly from one URL to
another:

  \E]8;;http://123\e\\First URL\E]8;;http//456\e\\Second URL\E]8;;\e\\

This use-case *is* handled by term_osc8_open().

term_osc8_close() uses the information from term_osc8_open() to add
per-row URL data (using the new ‘extra’ row data).

terminal.h

@@ -689,3 +689,6 @@ void term_ime_set_cursor_rect(
 
 void term_urls_reset(struct terminal *term);
 void term_collect_urls(struct terminal *term);
+
+void term_osc8_open(struct terminal *term, const char *uri);
+void term_osc8_close(struct terminal *term);