Add test-only API shim (Lite SDK)

[schmidt-sebastian]

This PR adds the API shim that I am using to run the existing integration tests agains the tree-shakeable API. It essentially exposes the tree-shakeable client in terms of the old API.

Same logic as #3361

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Binary Size Report

Affected SDKs

• @firebase/auth

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  main	177 kB	177 kB	+137 B (+0.1%)
  module	177 kB	177 kB	+137 B (+0.1%)

• @firebase/firestore

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  browser	247 kB	247 kB	+699 B (+0.3%)
  esm2017	193 kB	194 kB	+303 B (+0.2%)
  main	494 kB	495 kB	+1.27 kB (+0.3%)
  module	244 kB	245 kB	+667 B (+0.3%)
  react-native	194 kB	194 kB	+303 B (+0.2%)

• @firebase/firestore/exp

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  main	398 kB	400 kB	+1.74 kB (+0.4%)

• @firebase/firestore/lite

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  main	124 kB	124 kB	-73 B (-0.1%)

• @firebase/firestore/memory

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  browser	184 kB	185 kB	+385 B (+0.2%)
  esm2017	145 kB	145 kB	+138 B (+0.1%)
  main	363 kB	364 kB	+466 B (+0.1%)
  module	182 kB	183 kB	+363 B (+0.2%)
  react-native	145 kB	145 kB	+138 B (+0.1%)

• @firebase/functions

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  browser	9.18 kB	9.64 kB	+463 B (+5.0%)
  esm2017	7.01 kB	7.30 kB	+285 B (+4.1%)
  main	9.21 kB	9.67 kB	+463 B (+5.0%)
  module	9.00 kB	9.45 kB	+451 B (+5.0%)

• firebase

  Type	Base (064cf2e)	Head (f2772b5)	Diff
  firebase-auth.js	173 kB	174 kB	+133 B (+0.1%)
  firebase-firestore.js	285 kB	285 kB	+657 B (+0.2%)
  firebase-firestore.memory.js	224 kB	224 kB	+362 B (+0.2%)
  firebase-functions.js	9.60 kB	9.84 kB	+242 B (+2.5%)
  firebase.js	818 kB	819 kB	+1.04 kB (+0.1%)

Test Logs

• Base (064cf2e5): https://github.com/firebase/firebase-js-sdk/actions/runs/159906269
• Head (f2772b58): https://github.com/firebase/firebase-js-sdk/actions/runs/162836336

[thebrianchen]

s/private/private readonly

[thebrianchen]

Same minor/optional nits/questions as in #3361.