Convert 3 LOCALE_TRANSLATION_OVERWRITE_* constants used in the UI to a backed enum

Problem/Motivation

Surfaced by @alexpott while reviewing #3398982: ConfigFormBase + validation constraints: support non-1:1 form element-to-config property mapping again — it'd have been out of scope there.

Convert

/**
 * UI option for override of existing translations. Override any translation.
 */
const LOCALE_TRANSLATION_OVERWRITE_ALL = 'all';

/**
 * UI option for override of existing translations.
 *
 * Only override non-customized translations.
 */
const LOCALE_TRANSLATION_OVERWRITE_NON_CUSTOMIZED = 'non_customized';

/**
 * UI option for override of existing translations.
 *
 * Don't override existing translations.
 */
const LOCALE_TRANSLATION_OVERWRITE_NONE = 'none';

to a backed enum.

(These were introduced by #1804688: Download and import interface translations over a decade ago, when enums were a distant dream 😄.)

Steps to reproduce

N/A

Proposed resolution

Convert to backed enum.

Remaining tasks

• Add backed enum.
• Deprecate the old constants
• Reviews

User interface changes

None.

API changes

Deprecate the 3 constants.

Data model changes

None.

Release notes snippet

None.