Added assert not command (assertion that the value/expression is false) (#9235)

# Description
A new command to simplify assertions for `false`.

The name is just a draft, though I could not come up with a better name.
I have rejected `assert false`, because I would have to rename `assert`
to `assert true` which would break the compatibility and worsen the
shell experience of the good old `assert`.

Another idea I have rejected was something like `assert_false` to keep
it consistent with the naming convention of our stdlib.

I am open to suggestions :)

# User-Facing Changes
Just a new command

# Tests + Formatting
- [x] Done

# After Submitting
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crates/nu-std/tests/test_asserts.nu

@@ -7,6 +7,13 @@ export def test_assert [] {
     assert error { assert (1 + 2 == 4) }
 }
 
+export def test_assert_not [] {
+    assert not false
+    assert not (1 + 2 == 4)
+    assert error { assert not true }
+    assert error { assert not (1 + 2 == 3) }
+}
+
 export def test_assert_equal [] {
     assert equal (1 + 2) 3
     assert equal (0.1 + 0.2 | into string | into decimal) 0.3 # 0.30000000000000004 == 0.3

crates/nu-std/tests/test_logger.nu

@@ -114,8 +114,8 @@ def "assert custom message contains" [
     tested_str
 ] {
     let output = (run custom $system_level $format $message_level)
-    assert ($output | str contains  $tested_str)
-    assert ($output | str contains "test message")
+    assert str contains $output $tested_str
+    assert str contains $output "test message"
 }
 
 def "assert custom message not contains" [
@@ -125,8 +125,8 @@ def "assert custom message not contains" [
     tested_str
 ] {
     let output = (run custom $system_level $format $message_level)
-    assert (not ($output | str contains  $tested_str))
-    assert ($output | str contains "test message")
+    assert not ($output | str contains  $tested_str)
+    assert str contains $output "test message"
 }
 
 def "assert no custom message" [