Add --print and --format options

[friederbluemle]

Adds --print <platform> with an optional --format <template>. Without a template it lists the three native values in the style of --dry-run; with one, it fills the template, so a composed identifier such as a Sentry release string is a single call:

$ npx rn-version-sync --print ios
appId: com.example.app
versionName: 1.2.3
versionCode: 10203

$ npx rn-version-sync --print ios --format '{appId}@{versionName}+{versionCode}'
com.example.app@1.2.3+10203

• Placeholders are {appId}, {versionName} and {versionCode}, matching the library field names. An unknown placeholder fails with the available list; other braces pass through, so there are no escaping rules.
• Reads are lazy: only referenced placeholders are resolved, so a template without {appId} keeps working in projects with flavor-specific application ids where --print-app-id fails.
• Values are inserted as written in the native file, like the single-value --print-* flags. The character check stays specific to --print-env, where it protects eval.
• --configuration applies to {appId} for iOS as with the other flags. --format without --print is an error.
• The README documents the new flag and switches the Sentry example to the one-liner, with --print-env shown after it for scripts that need several values.

Internally the platform switch for reading the app id and versions moved into two helpers shared with readNativeValues; formatTemplate is exported alongside formatEnv.