Add step function for Tuya Zigbee curtains

[zhangshen001]

Check all that apply

Type of Change

• WWST Certification Request
  • If this is your first time contributing code:
    • I have reviewed the README.md file
    • I have reviewed the CODE_OF_CONDUCT.md file
    • I have signed the CLA
  • I plan on entering a WWST Certification Request or have entered a request through the WWST Certification console at developer.smartthings.com
• Bug fix
• New feature
• Refactor

Checklist

• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have verified my changes by testing with a device or have communicated a plan for testing
• I am adding new behavior, such as adding a sub-driver, and have added and run new unit tests to cover the new behavior

Description of Change

Summary of Completed Tests

• Co-Authored-By: Cline SR (50%)
• CGAI & AI Review

[CLAassistant]

All committers have signed the CLA.

[vangoran]

Let's add '_' into value and make same style with other static variables

[vangoran]

跟已有的全局变量保持一致，采用驼峰命名法 + 前缀‘_’

[partner-hub-chenxuelong]

For example, if the curtain is initially at level 30 and receives a +10 command, setting the target_level to 40, and then receives a +2 command when the actual curtain position reaches 35, the current_level might still be 30. If the current_level were used as the baseline and the target_level were set to 32, the motor would reverse. Therefore, the target_level is calculated based on the saved LATEST_TARGET_LEVEL and is set to 42.

[hcarter-775]

Hi @zhangshen001, would you please create a ticket for this PR? Thank you

[hongye-samsung]

Hi @zhangshen001, would you please create a ticket for this PR? Thank you

Hi~ @hcarter-775 , the task ticket is CHAD-18662 "Add statelessWindowShadeLevelStep support to the Zigbee Window Covering driver", because Tuya curtain driver is not in the path of "zigbee-window-treatment", So, we submitted this separate PR.