config+url: add support for user-defined regex patterns

Users can now define their own regex patterns, and use them via key
bindings:

    [regex:foo]
    regex=foo(bar)?
    launch=path-to-script-or-application {match}

    [key-bindings]
    regex-launch=[foo] Control+Shift+q
    regex-copy=[foo] Control+Mod1+Shift+q

That is, add a section called 'regex:', followed by an
identifier. Define a regex and a launcher command line.

Add a key-binding, regex-launch and/or regex-copy (similar to
show-urls-launch and show-urls-copy), and connect them to the regex
with the "[regex-name]" syntax (similar to how the pipe-* bindings
work).

terminal.h

@@ -789,6 +789,7 @@ struct terminal {
     bool urls_show_uri_on_jump_label;
     struct grid *url_grid_snapshot;
     bool ime_reenable_after_url_mode;
+    const struct config_spawn_template *url_launch;
 
 #if defined(FOOT_IME_ENABLED) && FOOT_IME_ENABLED
     bool ime_enabled;