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 })
    ```

test/functional/ui/float_spec.lua

@@ -168,6 +168,29 @@ describe('float window', function()
     eq(7, pos[2])
   end)
 
+  it('opened with correct position relative to the mouse', function()
+    meths.input_mouse('left', 'press', '', 0, 10, 10)
+    local pos = exec_lua([[
+      local bufnr = vim.api.nvim_create_buf(false, true)
+
+      local opts = {
+        width = 10,
+        height = 10,
+        col = 1,
+        row = 2,
+        relative = 'mouse',
+        style = 'minimal'
+      }
+
+      local win_id = vim.api.nvim_open_win(bufnr, false, opts)
+
+      return vim.api.nvim_win_get_position(win_id)
+    ]])
+
+    eq(12, pos[1])
+    eq(11, pos[2])
+  end)
+
   it('opened with correct position relative to the cursor', function()
     local pos = exec_lua([[
       local bufnr = vim.api.nvim_create_buf(false, true)