Rename decodeSessionToken to decodeIdToken

[jplhomer]

WHY are these changes introduced?

RE: https://github.com/Shopify/app-ui/issues/1619

We're renaming sessionToken to idToken everywhere.

As part of this, we want to rename decodeSessionToken to decodeIdToken in the @shopify/shopify-api package.

WHAT is this pull request doing?

• Our strategy is to provide a new future flag decodeIdToken which opts into removing the decodeSessionId function.
• The new decodeIdToken function is available immediately
• Consumers can opt-in to the new behavior where the old function is gone
• This will become the default at our next major version.

Type of change

• Minor: New feature (non-breaking change which adds functionality)

Checklist

• I have used pnpm changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)