[Fiona2016]

Release 0.4.0: bumps package.json to 0.4.0 and adds the 0.4.0 CHANGELOG section.

What ships (already on publish):

• sessionSampleRate and sessionOnError (✨ add session sampling and error-session capture (sessionOnError) #29)
• remote configuration: remoteConfigurationEnabled, getRemoteConfig() (✨ add remote configuration for the session sampling #30)
• renderer-bridge hardening: intake integer-path validation, prototype-name rejection, parse-failure telemetry without content; crash dumps kept when their report could not be written

All new options default to off / unchanged behaviour; the only change for an application that upgrades without touching its configuration is the _dd.configuration.session_sample_rate field on every RUM event.

After merge: tag the merge commit v0.4.0, run Publish with dry_run: true, then publish.