Bad nargs, Halstead and Mi metrics for JS arrow function

[marco-c]

(site) => site has reasonable values for the metrics, while site => site has bad values.

The left side is site => site, the right side is (site) => site:

[calixteman]

With x => x there is only one param:
https://github.com/tree-sitter/tree-sitter-javascript/blob/master/grammar.js#L678
and so we should get parameter instead of parameters:
https://github.com/mozilla/rust-code-analysis/blob/master/src/metrics/fn_args.rs#L75-L112

To avoid such inconsistency, we should likely add a s upstream.