Correctly propagate signals from cancelled jobs into parse_execution_context

This concerns code like the following:

    while true ; sleep 100; end

Here 'while' is a "simple block execution" and does not create a new job,
or get a pgid. Each 'sleep' however is an external command execution, and
is treated as a distinct job. (bash is the same way). So `while` and
`sleep` are always in different job groups.

The problem comes about if 'sleep' is cancelled through SIGINT or SIGQUIT.
Prior to 2a4c545b21, if *any* process got a SIGINT or SIGQUIT, then fish
would mark a global "stop executing" variable. This obviously prevents
background execution of fish functions.

In 2a4c545b21, this was changed so only the job's group gets marked as
cancelled. However in the case of one job group spawning another, we
weren't propagating the signal.

This adds a signal to parse_execution_context which the parser checks after
execution. It's not ideal since now we have three different places where
signals can be recorded. However it fixes this regression which is too
important to leave unfixed for long.

Fixes #7259

tests/pexpects/sigint.py

@@ -0,0 +1,25 @@
+#!/usr/bin/env python3
+from pexpect_helper import SpawnedProc
+
+sp = SpawnedProc()
+sendline, sleep, expect_prompt, expect_str = (
+    sp.sendline,
+    sp.sleep,
+    sp.expect_prompt,
+    sp.expect_str,
+)
+
+# Ensure that if child processes SIGINT, we exit our loops.
+# This is an interactive test because the parser is expected to
+# recover from SIGINT in interactive mode.
+# Test for #7259.
+
+expect_prompt()
+sendline("while true; sh -c 'echo Here we go; sleep .25; kill -s INT $$'; end")
+sleep(0.30)
+expect_str("Here we go")
+expect_prompt()
+
+sendline("echo it worked")
+expect_str("it worked")
+expect_prompt()