Fix database exp exports

[Feiyang1]

1. Export ServerValue as a value. Fixes Unclear how to use ServerValue.TIMESTAMP in RTDB SDK #4406
2. Remove implementation exports where only types should be exported.

[changeset-bot]

⚠️ No Changeset found

Latest commit: c331a22

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[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/database-exp

  Type	Base (5d7575b)	Head (61f1412)	Diff
  browser	277 kB	277 kB	-59 B (-0.0%)
  esm2017	241 kB	241 kB	-59 B (-0.0%)
  main	279 kB	279 kB	-158 B (-0.1%)
  module	277 kB	277 kB	-59 B (-0.0%)

Test Logs

• Base (5d7575b8): https://github.com/firebase/firebase-js-sdk/actions/runs/538551983
• Head (61f1412b): https://github.com/firebase/firebase-js-sdk/actions/runs/538885642

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (5d7575b) and head commit (61f1412).

Test Logs

• Base (5d7575b): https://github.com/firebase/firebase-js-sdk/actions/runs/538551983
• Head (61f1412): https://github.com/firebase/firebase-js-sdk/actions/runs/538885642