feat(float): open float relative to mouse #21531

Problem:
No easy way to position a LSP hover window relative to mouse.

Solution:
Introduce another option to the `relative` key in `nvim_open_win()`.

With this PR it should be possible to override the handler and do something
similar to this https://github.com/neovim/neovim/pull/19481#issuecomment-1193248674
to have hover information displayed from the mouse.

Test case:

    ```lua
    local util = require('vim.lsp.util')

    local function make_position_param(window, offset_encoding)
        window = window or 0
        local buf = vim.api.nvim_win_get_buf(window)
        local row, col

        local mouse = vim.fn.getmousepos()
        row = mouse.line
        col = mouse.column

        offset_encoding = offset_encoding or util._get_offset_encoding(buf)
        row = row - 1
        local line = vim.api.nvim_buf_get_lines(buf, row, row + 1, true)[1]
        if not line then
            return { line = 0, character = 0 }
        end
        if #line < col then
            return { line = 0, character = 0 }
        end

        col = util._str_utfindex_enc(line, col, offset_encoding)

        return { line = row, character = col }
    end

    local make_params = function(window, offset_encoding)
        window = window or 0
        local buf = vim.api.nvim_win_get_buf(window)
        offset_encoding = offset_encoding or util._get_offset_encoding(buf)
        return {
            textDocument = util.make_text_document_params(buf),
            position = make_position_param(window, offset_encoding),
        }
    end

    local hover_timer = nil
    vim.o.mousemoveevent = true

    vim.keymap.set({ '', 'i' }, '<MouseMove>', function()
        if hover_timer then
            hover_timer:close()
        end
        hover_timer = vim.defer_fn(function()
            hover_timer = nil
            local params = make_params()
            vim.lsp.buf_request(
                0,
                'textDocument/hover',
                params,
                vim.lsp.with(vim.lsp.handlers.hover, {
                    silent = true,
                    focusable = false,
                    relative = 'mouse',
                })
            )
        end, 500)
        return '<MouseMove>'
    end, { expr = true })
    ```

runtime/lua/vim/lsp/util.lua

@@ -1015,6 +1015,7 @@ end
 ---        - border (string or table) override `border`
 ---        - focusable (string or table) override `focusable`
 ---        - zindex (string or table) override `zindex`, defaults to 50
+---        - relative ("mouse"|"cursor") defaults to "cursor"
 ---@returns (table) Options
 function M.make_floating_popup_options(width, height, opts)
   validate({
@@ -1029,7 +1030,8 @@ function M.make_floating_popup_options(width, height, opts)
   local anchor = ''
   local row, col
 
-  local lines_above = vim.fn.winline() - 1
+  local lines_above = opts.relative == 'mouse' and vim.fn.getmousepos().line - 1
+    or vim.fn.winline() - 1
   local lines_below = vim.fn.winheight(0) - lines_above
 
   if lines_above < lines_below then
@@ -1042,7 +1044,9 @@ function M.make_floating_popup_options(width, height, opts)
     row = 0
   end
 
-  if vim.fn.wincol() + width + (opts.offset_x or 0) <= api.nvim_get_option('columns') then
+  local wincol = opts.relative == 'mouse' and vim.fn.getmousepos().column or vim.fn.wincol()
+
+  if wincol + width + (opts.offset_x or 0) <= api.nvim_get_option('columns') then
     anchor = anchor .. 'W'
     col = 0
   else
@@ -1062,7 +1066,7 @@ function M.make_floating_popup_options(width, height, opts)
     col = col + (opts.offset_x or 0),
     height = height,
     focusable = opts.focusable,
-    relative = 'cursor',
+    relative = opts.relative == 'mouse' and 'mouse' or 'cursor',
     row = row + (opts.offset_y or 0),
     style = 'minimal',
     width = width,