refactor(get,select,reject)!: deprecate --ignore-errors in favor of --optional (#16007)

# Description
As decided on the team meeting on 2025-06-19, rename `--ignore-errors
(-i)` to `--optional (-o)` with a (currently) indefinite grace period.

After `--ignore-errors (-i)` is removed, the short flag `-i` can be used
for `--ignore-case` (not implemented as of this PR)

# User-Facing Changes
`get`/`select`/`reject`: rename `--ignore-errors (-i)` to `--optional
(-o)` to better reflect its behavior.

# Tests + Formatting
- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
Update docs and inform third parties that integrate with nushell.

---------

Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>

crates/nu-command/tests/commands/default.rs

@@ -31,7 +31,7 @@ fn adds_row_data_if_column_missing() {
 
 #[test]
 fn default_after_empty_filter() {
-    let actual = nu!("[a b] | where $it == 'c' | get -i 0 | default 'd'");
+    let actual = nu!("[a b] | where $it == 'c' | get -o 0 | default 'd'");
 
     assert_eq!(actual.out, "d");
 }