Make variable value listbox searchable

What does this MR do and why?

#⃣ for #425052 (closed)

Make variable value listbox searchable

There's a GlDropdown in the UI to select prefilled variable values when running a manual pipeline - this MR:

• converts the GlDropdown to a GlCollapsibleListbox this was handled by !143733 (merged)
• moves it into its own component (VariableValuesListbox)
• makes it searchable so that values can be selected more easily using only the keyboard

MR acceptance checklist

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Screenshots or screen recordings

Before	After
Screen_Recording_2024-02-06_at_19.02.44	Screen_Recording_2024-02-06_at_18.57.53

How to set up and validate locally

1. configure some prefilled variable values in a project's pipeline:

# .gitlab-ci.yml

# sets up a variable called FLAG that defaults to "test"
# with options "dev", "qa", "test", and "prod"

variables:
  FLAG:
    value: "test"
    options:
      - "dev"
      - "qa"
      - "test"
      - "prod"
    description: "Variable description" # a variable description is required for the values dropdown to appear

2. navigate to the project, and in the sidebar choose Build => Pipelines
3. click the Run pipeline button in the upper right
4. the FLAG variable should show up as a row in the variables form, with a dropdown as the value field and the description below