Fuzz more realistically with keyword const eval (#15036)

# Description
The parsing logic for several of our keywords is conditional on the
particular commands for those keywords being in scope:


942030199d/crates/nu-parser/src/parse_keywords.rs (L272-L279)

Thus the following involved parsing logic was not fuzzed by the existing
`parse` fuzz target so far.

This adds an additional fuzz target `parse_with_keywords` that loads the
commands from `nu-cmd-lang`. Those are primarily the keyword
implementations, thus the relevant code paths in the parser that depend
on those `DeclId`s and the potential const eval of `if` etc. get
unlocked.

The existing `parse` target is preserved if you have concerns about the
fuzzing breaking containment in some form due to those commands.

# Tests + Formatting
Found https://github.com/nushell/nushell/issues/14972 with this target

crates/nu-parser/fuzz/fuzz_targets/parse_with_keywords.rs

@@ -0,0 +1,14 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+use nu_cmd_lang::create_default_context;
+use nu_parser::*;
+use nu_protocol::engine::StateWorkingSet;
+
+fuzz_target!(|data: &[u8]| {
+    let engine_state = create_default_context();
+    let mut working_set = StateWorkingSet::new(&engine_state);
+
+    let _block = parse(&mut working_set, None, &data, true);
+});