updated NU_LIB_DIRS delimiter for command line (#10837)

# Description

This PR corrects some help text by stating what the real delimiter is
for the `--include-path` command line parameter which is `char
record_sep` aka `\x1e`. Up to this point, this has really only been used
for the vscode extension to setup the NU_LIB_DIRS env var correctly. We
tried `:` and `;` and neither would work so we had to choose something
that wouldn't be confused so easily.

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src/command.rs

@@ -262,7 +262,7 @@ impl Command for Nu {
             .named(
                 "include-path",
                 SyntaxShape::String,
-                "set the NU_LIB_DIRS for the given script (semicolon-delimited)",
+                "set the NU_LIB_DIRS for the given script (delimited by char record_sep ('\x1e'))",
                 Some('I'),
             )
             .switch("interactive", "start as an interactive shell", Some('i'))