Add a flag harmony_trailing_commas_in_parameters that allows trailing
commas in function parameter declaration lists and function call
parameter lists. Trailing commas are allowed in parenthetical lists like
`(a, b, c,)` only if the next token is `=>`, thereby making it an arrow
function declaration. Only 1 trailing comma is allowed, not `(a,,)`. A
trailing comma must follow a non-rest parameter, so `(,)` and `(...a,)`
are still SyntaxErrors. However, a trailing comma is allowed after a
spread parameter, e.g. `a(...b,);`.

Add parser tests for all of the above.

BUG=v8:5051
LOG=y

Review-Url: https://codereview.chromium.org/2094463002
Cr-Commit-Position: refs/heads/master@{#37355}