[DX] Add recommended extensions and direction for sh / yml formatting

[Ikuni17]

It would be good to get some consistent formatting and linting on specific file types, namely .yml and .sh. For example, I see PRs which will flip .yml from single to double quote and/or adjust spacing. Then another PR comes along doing the opposite, due to the user's environment. This does increase the diff, or require Save without Formatting, and/or manually formatting. In the case of yml it can cause parsing issues which break the file too.

Ideas:

• Can we utilize .editorconfig better?
• Should we add shellcheck?
• Can we nudge developers in the right direction with recommended extensions?
• Should we add a Checks step to be more firm about these changes?

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)