Increment SHLVL before run_repl() (#14732)

# Description

A follow-on to #14727:

* Instead of using `is-interactive` as the trigger for incrementing
`SHLVL`, this change puts the increment logic just before `run_repl()`
is called.
* Tests are changed to use `-e`
* Moves the `confirm_stdin_is_terminal()` call immediately **after** the
`prerun_cmd` (which executes `--execute (-e) <commandstring>`. The fact
that it was **before** that call seems to be a bug, since the error
message says *"or provide arguments to invoke a script"* even if
`--execute` was used. This change enables REPL testing using `--execute
(-e)`.
* Added a test to ensure `-c` does *not* increment SHLVL.

# User-Facing Changes

`$env.SHLVL` runs before the REPL is started, rather than when
`is-interactive`

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-cli/src/repl.rs

@@ -63,8 +63,6 @@ pub fn evaluate_repl(
     let config = engine_state.get_config();
     let use_color = config.use_ansi_coloring.get(engine_state);
 
-    confirm_stdin_is_terminal()?;
-
     let mut entry_num = 0;
 
     // Let's grab the shell_integration configs
@@ -104,6 +102,8 @@ pub fn evaluate_repl(
         engine_state.merge_env(&mut unique_stack)?;
     }
 
+    confirm_stdin_is_terminal()?;
+
     let hostname = System::host_name();
     if shell_integration_osc2 {
         run_shell_integration_osc2(None, engine_state, &mut unique_stack, use_color);