add function --shadow-builtin flag

It's currently too easy for someone to bork their shell by doing something
like `function test; return 0; end`. That's obviously a silly, contrived,
example but the point is that novice users who learn about functions are
prone to do something like that without realizing it will bork the shell. Even
expert users who know about the `test` builtin might forget that, say, `pwd`
is a builtin.

This change adds a `--shadow-builtin` flag that must be specified to
indicate you know what you're doing.

Fixes #3000

tests/function.err

@@ -0,0 +1,8 @@
+function: function name shadows a builtin so you must use '--shadow-builtin'
+fish: function pwd; end
+      ^
+yes, it failed as expected
+function: function name does not shadow a builtin so you must not use '--shadow-builtin'
+fish: function not_builtin --shadow-builtin; end
+      ^
+yes, it failed as expected