feat(pickers): improve center strategy by adding results_height option

[alissonbrunosa]

Description

I've been using this patched version of the center layout strategy for a while and thought it was worth proposing upstream. It introduces a new layout option, results_height, allowing users to explicitly set the height of the results panel.

In addition, this refactors how window dimensions and positions are calculated, properly centring the entire layout (including preview, results, and prompt), rather than just the prompt + results block.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

I've tested this change with different configurations, such as mirror, prompt_position, height and with the new option results_height

Dropdown theme

pickers = {
    find_files = {
        theme = "dropdown",
    }
}

with mirror = true

pickers = {
    find_files = {
        theme = "dropdown",
       layout_config = {
            mirror = true,
       },
    }
}

with results_height = 5

pickers = {
    find_files = {
        theme = "dropdown",
       layout_config = {
            results_height = 5,
       },
    }
}

with results_height = 5 and mirror = true

pickers = {
    find_files = {
        theme = "dropdown",
       layout_config = {
            results_height = 5,
            mirror = true,
       },
    }
}

with prompt_position = 'bottom'

pickers = {
    find_files = {
        theme = "dropdown",
       layout_config = {
            results_height = 5,
            mirror = true,
            prompt_position = 'bottom',
       },
    }
}

with height = 40 and results_height = 10

pickers = {
    find_files = {
        theme = "dropdown",
       layout_config = {
            height = 10
            results_height = 10,
       },
    }
}

Configuration

• Neovim version (nvim --version):

NVIM v0.10.4
Build type: RelWithDebInfo
LuaJIT 2.1.1720049189
Run "nvim -V1 -v" for more info

• Operating system and version: Fedora 41

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)

[alissonbrunosa]

[ Note 🗒️ ]
Since the height option now sets the overall height and not just the prompt
+ results, we need to make it a bit taller and set the results_height to 15
to replicate the previous layout