Attempt to handle automatic conflict resolution for local backports

[spalger]

sorenlouv/backport#206 added support for automatic conflict resolution logic in the backport script, which we should try to take advantage of.

Ideally this could be applied via the Github Action at some point, but for now I think it's reasonable to just focus on handling these cases when running backports locally where failures can be debugged easier with access to the filesystem.

My initial plan is to implement resolution for two types of conflicts:

• yarn.lock run BUILD_TS_REFS_DISABLE=true yarn kbn bootstrap
• api_docs/**/* run node scripts/build_api_docs

[elasticmachine]

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