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>

tests/parsing/samples/recursive_func_with_alias.nu

@@ -10,7 +10,7 @@ export def "update" [
     let input = $in
     match ($input | describe | str replace --regex '<.*' '') {
         record => {
-            if ($input | get -i $field) != null {
+            if ($input | get -o $field) != null {
                 $input | orig update $field $value
             } else { $input }
         }