mention psub in the error for "Invalid redirection target"

Currently fish errors out with

```fish
fish: Invalid redirection target:
rev <(ls)
    ^~~~^
```

This isn't very helpful in telling the user what they could be doing instead:
`rev (ls | psub)`.

Closes #11287

src/parse_execution.rs

@@ -33,7 +33,10 @@
     WILDCARD_ERR_MSG,
 };
 use crate::parse_tree::{LineCounter, NodeRef, ParsedSourceRef};
-use crate::parse_util::parse_util_unescape_wildcards;
+use crate::parse_util::{
+    parse_util_locate_cmdsubst_range, parse_util_unescape_wildcards,
+    MaybeParentheses::CommandSubstitution,
+};
 use crate::parser::{Block, BlockData, BlockId, BlockType, LoopStatus, Parser, ProfileItem};
 use crate::parser_keywords::parser_keywords_is_subcommand;
 use crate::path::{path_as_implicit_cd, path_try_get_path};
@@ -1471,9 +1474,10 @@ fn determine_redirections(
                 ctx,
                 None,
             );
+
             if !target_expanded || target.is_empty() {
                 // TODO: Improve this error message.
-                return report_error!(
+                let error_ret = report_error!(
                     self,
                     ctx,
                     STATUS_INVALID_ARGS,
@@ -1481,6 +1485,23 @@ fn determine_redirections(
                     "Invalid redirection target: %ls",
                     target
                 );
+                if oper.mode == RedirectionMode::input && {
+                    let redir_unexpanded = self.node_source(redir_node);
+                    redir_unexpanded.starts_with(L!("<("))
+                        && match parse_util_locate_cmdsubst_range(
+                            &redir_unexpanded[1..],
+                            &mut 0,
+                            false,
+                            None,
+                            None,
+                        ) {
+                            CommandSubstitution(p) => p.start() == 0,
+                            _ => false,
+                        }
+                } {
+                    eprintf!("If you wish to use process substitution, consider the psub command, see: `help psub`\n");
+                }
+                return error_ret;
             }
 
             // Make a redirection spec from the redirect token.

tests/checks/psub.fish

@@ -59,3 +59,22 @@ end
 
 set -l diffs (comm -3 (__fish_print_help psub 2>| psub) (psub -hs banana 2>| psub))
 test -z "$diffs"
+
+# In cases that look like process substitutions, mention psub.
+
+echo <(seq 0)
+# CHECKERR: {{.*}}/psub.fish (line {{\d+}}): Invalid redirection target:
+# CHECKERR: echo <(seq 0)
+# CHECKERR:      ^~~~~~~^
+# CHECKERR: If you wish to use process substitution, consider the psub command, see: `help psub`
+
+# To-do: should also mention psub here.
+echo <(seq 1)
+# CHECKERR: warning: An error occurred while redirecting file '1'
+# CHECKERR: warning: Path '1' does not exist
+
+echo <(seq 2)
+# CHECKERR: {{.*}}/psub.fish (line {{\d+}}): Invalid redirection target:
+# CHECKERR: echo <(seq 2)
+# CHECKERR:      ^~~~~~~^
+# CHECKERR: If you wish to use process substitution, consider the psub command, see: `help psub`