[ts/build_ts_refs] add support for --clean flag

[spalger]

Does a tiny bit of refactoring in the dev/typescript directory to implement a --clean flag for node scripts/build_ts_refs which clears out the outDir for each TS project referenced from the root tsconfig.refs.json file.

[elasticmachine]

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

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 5715246

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[mistic]

LGTM

[kibanamachine]

Backport result

{"level":"info","message":"POST https://api.github.com/graphql (status: 200)"}
{"level":"info","message":"POST https://api.github.com/graphql (status: 200)"}
{"meta":{"labels":["Team:Operations","auto-backport","release_note:skip","v7.12.0","v8.0.0"],"branchLabelMapping":{"^v8.0.0$":"master","^v7.12.0$":"7.x","^v(\\d+).(\\d+).\\d+$":"$1.$2"},"existingTargetPullRequests":[]},"level":"info","message":"Inputs when calculating target branches:"}
{"meta":["7.x"],"level":"info","message":"Target branches inferred from labels:"}
{"meta":{"killed":false,"code":2,"signal":null,"cmd":"git remote rm kibanamachine","stdout":"","stderr":"error: No such remote: 'kibanamachine'\n"},"level":"info","message":"exec error 'git remote rm kibanamachine':"}
{"meta":{"killed":false,"code":2,"signal":null,"cmd":"git remote rm elastic","stdout":"","stderr":"error: No such remote: 'elastic'\n"},"level":"info","message":"exec error 'git remote rm elastic':"}
{"level":"info","message":"Backporting [{\"sourceBranch\":\"master\",\"targetBranchesFromLabels\":[\"7.x\"],\"sha\":\"c0a974aa0e048af7c5829172c41aa32d860961c4\",\"formattedMessage\":\"[ts/build_ts_refs] add support for --clean flag (#91060)\",\"originalMessage\":\"[ts/build_ts_refs] add support for --clean flag (#91060)\\n\\nCo-authored-by: spalger <[email protected]>\",\"pullNumber\":91060,\"existingTargetPullRequests\":[]}] to 7.x"}

Backporting to 7.x:
{"level":"info","message":"Backporting via filesystem"}
{"level":"info","message":"Creating PR with title: \"[7.x] [ts/build_ts_refs] add support for --clean flag (#91060)\". kibanamachine:backport/7.x/pr-91060 -> 7.x"}
{"level":"info","message":"POST /repos/elastic/kibana/pulls - 201 in 1227ms"}
{"level":"info","message":"Adding assignees to #91078: spalger"}
{"level":"info","message":"POST /repos/elastic/kibana/issues/91078/assignees - 201 in 481ms"}
{"level":"info","message":"Adding labels: backport"}
{"level":"info","message":"POST /repos/elastic/kibana/issues/91078/labels - 200 in 330ms"}
View pull request: https://github.com/elastic/kibana/pull/91078