Introduce webext-sdk dependency

Background / User story

In order to be able to switch to the SDK (see #972 (closed)), we need to add webext-sdk as a new dependendy so that we can start using it.

What to change

• Design: N/A
• Research: N/A
• Spec: N/A
• Legal: N/A
• Development:
  • Add vendor/webext-sdk submodule.
  • Update npm configuration to run npm install from repository's root directory.
  • Update READMEs and GitLab CI configuration accordingly.

Hints for testers

• Extension builds (with and without snippets) should continue to generate successfully, both locally and in the CI.
  • Building the extension should also cause SDK bundles to be generated in the vendor/webext-sdk/dist/ directory, but they should not be included in extension builds.
• It should continue to be possible to follow the extension build instructions in the README.
• The webext-sdk git submodule should be resolved in the vendor/ directory.

Hints for translators

N/A