fix: add setBindings to the browser logger - #2471

Fixes #2021

Problem

The browser logger (browser.js) never implemented setBindings, even though the TypeScript types declare it on LoggerExtras — so any code path that resolves the browser field of package.json throws TypeError: logger.setBindings is not a function. (To answer the question in the issue thread: it's a JS bug — a missing method in the browser implementation, not a typings problem. The named/default import distinction in the report was a red herring; both exports point at the same function, what differs is whether the bundler picked pino.js or browser.js.)

Fix

Implement setBindings on the browser logger:

• merges the new bindings into the logger's bindings (children inherit the method through the prototype chain and merge into their own bindings),
• mirrors the new bindings into _logEvent.bindings so browser.transmit receives them, matching what child() does,
• rebuilds the level functions (same mechanism as the level setter) so subsequent log calls prepend the updated bindings.

Consistent with Node semantics: like asChindings in lib/proto.js, updating a parent's bindings does not retroactively affect already-created children.

Testing

New test/browser-set-bindings.test.js (tape, same style as the other browser tests): method exists, bindings appear in subsequent logs, repeated calls merge, child loggers merge with their own bindings, works with customLevels (the issue's repro), and bindings are transmitted. All existing browser tests and lint pass.