Add initial value to reduce function for tab widths

[fastjames]

Description
When calculating total tab width, we recently saw a case where there
were no tabs and thus no IV for reduce. This adds a default starting
value of 0 which prevents a js exception in that case. I checked the rest of the js and found no other occurrences of a reduce call without an initial value.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)

[fastjames]

Edited desc to be clear: I have not added tests to this for two reasons:

1. The addition of a default initial value of 0 for a summation function seems pretty obvious
2. I didn't see much if any existing test code for javascript in this area, so I would have to start from scratch

[ericsaupe]

LGTM, thanks!

[spaghetticode]

@fastjames can you please rebase with master, so we get a green CI? thank you 🙏

[fastjames]

Rebased and force-pushed.

[kennyadsl]

Thanks!