Remove deprecated .extend transformation pattern

[renatonouman]

This PR makes styled-components .extend pattern optional, since it has been deprecated in v4(styled-components/styled-components#1908).

Moreover, appending .withConfig to other extend methods might break unrelated code, i.e. #33

Related to #47, #33 and #48

From #48

• make extend identifier configurable like other identifiers;
• set default value to an empty array;
• add readme/release notes with breaking change information and instructions to re-enable by providing settings.

[renatonouman]

@Igorbek any instructions on how to add this to readme and release notes?

[Igorbek]

Thank you for the contribution! Could you please add a line to the 'unreleased' section to the CHANGELOG.md file. I just created it.

[renatonouman]

Thank you for reviewing this so quickly. Changes made as requested!

[Igorbek]

Sorry, @renatonouman another pr got merged, please resolve conflicts and update snapshots.

[renatonouman]

Done @Igorbek

[renatonouman]

Glad to contribute! Thanks for the review and merge!

[renatonouman]

Hey @Igorbek! I just want to check with you when do you plan on releasing the next version?
Thanks!

[Igorbek]

I started preparing it lately, but got interrupted with other stuff, as usual. I'll try to release it tonight. Sorry for inconvenience.

[Igorbek]

It's now been released https://github.com/Igorbek/typescript-plugin-styled-components/releases/tag/1.5.0
Thank you for the contribution @renatonouman!

[renatonouman]

Thank you for the release @Igorbek.
Using the new version in my project already!