Rework exit command

Prior to this fix, the `exit` command would set a global variable in the
reader, which parse_execution would check. However in concurrent mode you
may have multiple scripts being sourced at once, and 'exit' should only
apply to the current script.

Switch to using a variable in the parser's libdata instead.

src/parser.h

@@ -182,11 +182,19 @@ struct library_data_t {
     bool suppress_fish_trace{false};
 
     /// Whether we should break or continue the current loop.
+    /// This is set by the 'break' and 'continue' commands.
     enum loop_status_t loop_status { loop_status_t::normals };
 
     /// Whether we should return from the current function.
+    /// This is set by the 'return' command.
     bool returning{false};
 
+    /// Whether we should stop executing.
+    /// This is set by the 'exit' command, and unset after 'reader_read'.
+    /// Note this only exits up to the "current script boundary." That is, a call to exit within a
+    /// 'source' or 'read' command will only exit up to that command.
+    bool exit_current_script{false};
+
     /// The read limit to apply to captured subshell output, or 0 for none.
     size_t read_limit{0};