Use named export of @storybook/addons

[inokawa]

Hi,
In storybook v7, default export of @storybook/addons will be removed.
https://github.com/storybookjs/storybook/blob/next/MIGRATION.md#no-more-default-export-from-storybookaddons

When I run storycap in 7.0.0-alpha.26, I got this error. This PR would fix it.

register.js:8 Uncaught TypeError: addons_1.default.register is not a function
    at node_modules/storycap/lib/client/register.js (register.js:8:18)
    at __require (register.mjs:9:50)
    at node_modules/storycap/register.js (register.js:1:18)
    at __require (register.mjs:9:50)
    at register.js:1:18
node_modules/storycap/lib/client/register.js @ register.js:8
__require @ register.mjs:9
node_modules/storycap/register.js @ register.js:1
__require @ register.mjs:9
（匿名） @ register.js:1