Fix some issues with builtin.colorscheme

[ronandalton]

Description

This PR fixes a couple of issues I found with the current implementation of builtin.colorscheme. Currently two problems can be observed:

• The color scheme changes when searching, even when enable_preview is false. Shifting the selection behaves as expected however.
• Color schemes cannot be previewed by clicking with the mouse when enable_preview is set to true.

The code is modified to fix these issues.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Test A:
  1. Run :Telescope colorscheme enable_preview=false
  2. Start searching for something
  3. Note that the color scheme is not changed until the enter key is pressed
• Test B
  1. Run :Telescope colorscheme enable_preview=true
  2. Use the mouse to click on one of the options
  3. Observe that the color scheme is changed
  4. Exit Telescope and the original color scheme should be restored

Configuration:

• Neovim version (nvim --version): v0.10.1
• Operating system and version: Linux 6.9.3

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[ronandalton]

Note that I couldn't find any existing issues documenting these problems, which is why there are no issues linked. I can create them if that's a requirement.

[jamestrew]

Thanks!