[@svelteui/core]: add Flex component

[khalibloo]

Description

Added Flex component.

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing guide
• [x] Prefix your PR title with [@svelteui/core], [@svelteui/actions], [@svelteui/motion], [@svelteui/core], [core], or [docs].
• [x] Include a description of the changes made in the PR description and in the commit messages.
• [x] Include screenshots/videos of the changes made.
• [x] Verify that the linter and tests are passing, with yarn lint and yarn test or just run yarn prepush.

[khalibloo]

I must have accidentally sucked in a stray commit 😛 only noticed during prepush. Feel free to squash the commits if needed

[BeeMargarida]

Closes https://github.com/svelteuidev/svelteui/issues/254