refactor(alert): follow actual guideline

[davidrus]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines: https://github.com/wc-catalogue/blaze-elements/blob/master/docs/CONTRIBUTING.md#commit
• [x] There are no linting errors and code is properly formatted (your run before push yarn ts:format && yarn ts:lint:fix)
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here) Alert is not refactored

What is the new behavior? Add decorators, define component, importing, exporting are now in index

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

affects: @blaze-elements/alert

Closes #243