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-std/tests/logger_tests/test_basic_commands.nu

@@ -11,7 +11,7 @@ def run [
     } else {
         ^$nu.current-exe --no-config-file --commands $'use std; use std/log; NU_LOG_LEVEL=($system_level) log ($message_level) "test message"'
     }
-    | complete | get --ignore-errors stderr
+    | complete | get --optional stderr
 }
 
 def "assert no message" [