infrastructure: update "engines" to node>=10.9

[ljqx]

Last update of "engines" is about 1 year ago: 3d182f1

Now Node 10 is the oldest LTS in maintenance, the maintenance would end at 2021-04-30, which is also pretty close.

For adding support of BigInt #1988 and #2035, it's better that the environment has support of BigInt. Or the value of BigIntLiteral would be null for Node < 10.4, which increases maintenance cost.

And according to https://node.green/, Node 10.9 is the first version that fully supports ES6. We can generate ES6 code instead ES5.