`unsafe-eval` was needed in the extension Content Security
Policy to Wasm compilation in Chrome.

This CSP setting causes the extension to get flagged in the
Mozilla Add-On Marketplace, which discourages the use of
`unsafe-eval`.

However, Chrome has a `wasm-eval` CSP setting which also allows
extensions to compile Wasm without requiring `unsafe-eval`.
Inject this into the extension manifest when building the Chrome
extension.

Eventually this may change to `wasm-unsafe-eval` as drafted by
the CSP spec and be required by all browsers.