fix(posthog-js): use SPDX expression for license field

[jleknes]

Fixes #2627.

Replace "license": "SEE LICENSE IN LICENSE" in packages/browser/package.json with the SPDX expression "(Apache-2.0 AND MIT)". This unblocks automated license-compliance tools (e.g. license-checker-rseidelsohn) that first try to parse the license field as an SPDX expression and only fall back to LICENSE-file scanning when that fails – and the fallback can't reliably resolve a multi-license LICENSE file.

The expression uses AND because the LICENSE file applied to this package combines:

• Apache-2.0 – PostHog / Mixpanel original code
• MIT – code derived from Sentry, rrweb (and via vendored modules: Meta Metro, Expo)

AND (rather than OR) is the correct operator: a consumer of the published package must comply with both licenses simultaneously.

The expression validates against spdx-expression-parse (the same parser npm and most license-checker tools use).

Out of scope for this PR (worth a follow-up)

• @posthog/rollup-plugin and @posthog/webpack-plugin are missing the license field entirely.
• The other publishable sub-packages currently declare "MIT". Some of them (notably posthog-react-native, @posthog/core, posthog-node) also include MIT-licensed third-party derivatives sitting alongside PostHog's own code, so it's worth confirming whether "MIT" is accurate for each or whether they should also use a (Apache-2.0 AND MIT) expression.

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[greptile-apps]

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