fix: show custom option in easing dropdow only when used

[JayaKrishnaNamburu]

fixes #4197

Description

This PR improves the UX for custom option in the easing functions. Currently it is displaying every time, with the PR we show them only when value added by the users are custom and hides it when the entered value doesn't come under custom category.

Steps for reproduction

• Add a transition on a node and then select a value from the dropdown.
• When the dropdown is open and the easing that is added is a valid one. The Custom option should be hidden.
• When a custom value is added under the text-area, then the select box should show Custom value.

Screen.Recording.2024-10-15.at.12.37.09.PM.mov

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review

Before merging

• tested locally and on preview environment (preview dev login: 5de6)

[kof]

looking great!