Don't overwrite unrelated variables with for-loop-variables

for-loops that were not inside a function could overwrite global
and universal variables with the loop variable.  Avoid this by making
for-loop-variables local variables in their enclosing scope.

This means that if someone does:

    set a global
    for a in local; end
    echo $a

The local $a will shadow the global one (but not be visible in child
scopes). Which is surprising, but less dangerous than the previous
behavior.

The detection whether the loop is running inside a function was failing
inside command substitutions. Remove this special handling of functions
alltogether, it's not needed anymore.

Fixes #6480

tests/checks/for.fish

@@ -0,0 +1,34 @@
+# RUN: %fish %s
+
+# A for-loop-variable is a local variable in the enclosing scope.
+set -g i global
+# implicit set -l i $i
+for i in local
+end
+set -ql i && echo $i
+# CHECK: local
+
+# The loop variable is initialized with any previous value.
+set -g j global
+for j in
+end
+set -ql j && echo $j
+# CHECK: global
+
+# Loop variables exist only locally in the enclosing local scope.
+# They do not modify other local/global/universal variables.
+set -g k global
+begin
+    for k in local1
+        echo $k
+        # CHECK: local1
+        for k in local2
+        end
+        echo $k
+        # CHECK: local2
+    end
+    echo $k
+    # CHECK: local1
+end
+echo $k
+# CHECK: global