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/format_conversions/csv.rs

@@ -326,7 +326,7 @@ fn from_csv_text_with_missing_columns_to_table() {
             r#"
                 open los_tres_caballeros.txt
                 | from csv --flexible
-                | get -i rusty_luck
+                | get -o rusty_luck
                 | compact
                 | length
             "#

crates/nu-command/tests/format_conversions/tsv.rs

@@ -245,7 +245,7 @@ fn from_tsv_text_with_missing_columns_to_table() {
             r#"
                 open los_tres_caballeros.txt
                 | from tsv --flexible
-                | get -i rusty_luck
+                | get -o rusty_luck
                 | compact
                 | length
             "#