support bigint cell content type

[ItsNickBarry]

The existing type casting (String(value)) also works for BigInts, so I've added support in the Typescript definitions and the JS type checks.

[speedytwenty]

Thanks for the PR(s)! I'd like to see a supporting test that simply outputs a bigint in cell content and the resulting output is as expected. This would be more to avoid regressions when refactoring.

I'll create a new release next week. If you haven't added a test, I'll accept this as is.

[speedytwenty]

You'll find basic examples for issue based tests here: https://github.com/cli-table/cli-table3/tree/master/test/issues

[ItsNickBarry]

Added a test. Also had to update the eslint ecmaVersion so that it could parse bigint literals.

[speedytwenty]

Unsure whether this should be considered a "breaking change". If it is, a major release still seems like overkill.

[speedytwenty]

Thanks for your efforts. Tests look good. Your placement of them (within cell-test.js is also better than what I suggested).