chore: target ES2018 in packages

[trivikr]

Issue #, if available:
Refs: #1559

Description of changes:
target ES2018 in packages

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[codecov-io]

Codecov Report

Merging #1562 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1562      +/-   ##
==========================================
+ Coverage   79.84%   79.87%   +0.03%     
==========================================
  Files         304      312       +8     
  Lines       11753    11843      +90     
  Branches     2511     2503       -8     
==========================================
+ Hits         9384     9460      +76     
- Misses       2369     2383      +14     

Impacted Files	Coverage Δ
...-provider-cognito-identity/src/IndexedDbStorage.ts	6.52% <0.00%> (-12.63%)	⬇️
packages/signature-v4/src/moveHeadersToQuery.ts	90.00% <0.00%> (-10.00%)	⬇️
packages/smithy-client/src/client.ts	76.47% <0.00%> (-7.74%)	⬇️
...kages/fetch-http-handler/src/fetch-http-handler.ts	9.52% <0.00%> (-6.76%)	⬇️
...s/middleware-bucket-endpoint/src/configurations.ts	95.65% <0.00%> (-4.35%)	⬇️
...ventstream-marshaller/src/EventStreamMarshaller.ts	91.66% <0.00%> (-4.34%)	⬇️
packages/xml-builder/src/XmlNode.ts	88.00% <0.00%> (-4.31%)	⬇️
packages/middleware-ssec/src/index.ts	82.60% <0.00%> (-2.58%)	⬇️
...dleware-bucket-endpoint/src/bucketHostnameUtils.ts	96.42% <0.00%> (-1.91%)	⬇️
...-sdk-transcribe-streaming/src/websocket-handler.ts	78.33% <0.00%> (-1.67%)	⬇️
... and 104 more

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[alexforsyth]

LGTM!

[github-actions]

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