disallow blocks as first-class values (#9587)

# Description

This PR disallows blocks as first-class values by removing the ability
to create them using the `block` syntax shape or type. Now, the parser
will only ever be able to create closures as first-class values.

This means that `{ 3 }` will always be treated as a closure, unless used
in the specifically supported use case of the literal being given as an
arg to `for`, `if` and other built-in block users.

Note: first-class value here means "value that can be passed into
commands and held in variables"

# User-Facing Changes

This may break some user scripts as `: block` is no longer allows as a
type annotation. Note: these cases were not actually supported before,
as, for example, passing a block that accessed a variable would have
errored when the block was later evaluated.

Closures do not have the restriction mentioned above and are the much
safer value to pass as first-class, so now they are the only block-like
value to be allowed to be passed.

# Tests + Formatting
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src/tests/test_type_check.rs

@@ -43,3 +43,19 @@ fn date_plus_duration() -> TestResult {
     let expected = "2023-04-20";
     run_test(input, expected)
 }
+
+#[test]
+fn block_not_first_class_def() -> TestResult {
+    fail_test(
+        "def foo [x: block] { do $x }",
+        "Blocks are not support as first-class values",
+    )
+}
+
+#[test]
+fn block_not_first_class_let() -> TestResult {
+    fail_test(
+        "let x: block = { 3 }",
+        "Blocks are not support as first-class values",
+    )
+}