Review and resolve open pull requests in upstream ExcelJS main repo

[protobi-pieter]

There are a large number of outstanding pull requests on the main ExcelJS repo https://github.com/exceljs/exceljs/pulls

It would be useful to review and resolve these. The following analysis reviews all open PRs and groups them based on

• complexity (easy vs hard),
• completeness (PR includes tests, examples, etc.?),
• salience (number of people commenting, number of comments, importance of topic)
• type (e.g. bug/enhancement/new)

https://github.com/protobi/exceljs/wiki/Analysis-of-open-pull-requests-in-ExcelJS