Suppress column index for default cal output (#13188)

# Description

* As discussed in the comments in #11954, this suppresses the index
column on `cal` output. It does that by running `table -i false` on the
results by default.
* Added new `--as-table/-t` flag to revert to the old behavior and
output the calendar as structured data
* Updated existing tests to use `--as-table`
* Added new tests against the string output
* Updated `length` test which also used `cal`
* Added new example for `--as-table`, with result

# User-Facing Changes

## Breaking change

The *default* `cal` output has changed from a `list` to a `string`. To
obtain structured data from `cal`, use the new `--as-table/-t` flag.

# Tests + Formatting

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


# After Submitting
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documentation](https://github.com/nushell/nushell.github.io) after the
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crates/nu-command/tests/commands/length.rs

@@ -2,7 +2,7 @@ use nu_test_support::nu;
 
 #[test]
 fn length_columns_in_cal_table() {
-    let actual = nu!("cal | columns | length");
+    let actual = nu!("cal --as-table | columns | length");
 
     assert_eq!(actual.out, "7");
 }