Consider making Xgettext.parseOptions configurable

[ryerh]

Xgettext.parseOptions are hard coded here as following:

parser = new Xgettext( {
    keywords: parserKeywords,
    parseOptions: {
        plugins: [ 
            'jsx',
            'classProperties',
            'objectRestSpread',
            'exportExtensions',
            'trailingFunctionCommas',
            'asyncFunctions',
        ],
        allowImportExportEverywhere: true,
    }
} );

The plugins part is missing some new ES syntax like:

• Exponentiation Operator const GB = 2 ** 10
• Dynamic import import('./foo.js').then()

I propose two approach to extend this feature:

1. Read parseOptions from $PWD/package.json -> i18nCalypso.parseOptions, then merge with default parseOptions
2. Extend existing parseOptions to keep update with new ES syntax

Any suggestions? I will open a PR after once an approach has been chosen.

[Yuyz0112]

+1 And xgettext-js has the babylon@6.8.4 dependency, which does not support some plugins like dynamicImport etc.