Fix #238 out of bounds crash when formatting tables

[jrturton]

Bug/issue #, if applicable: #238

Summary

If a table included multiple pipe characters that took the total number of columns (including spanning columns) above the main column count, this would cause a crash instead of just dropping the excess content as per the spec.

This PR removes the attempted out-of-bounds read of the widths array by capping the count.

Dependencies

None

Testing

Two unit tests have been added to exercise the code. To manually test it, you can attempt to parse markdown like this:

|A|B|C|
|-|-|-|
|1|2|3||4|5|6|

And then format the resulting document, which will crash without these changes in place.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary (not applicable)

[d-ronnqvist]

FYI: I think this can also be written as

Suggested change

	let lastColumn = min(finalColumnWidths.count, column + colspan)
	return (column..<lastColumn).map({ finalColumnWidths[$0] }).reduce(0, { $0 + $1 })
	finalColumnWidths.dropFirst(column).prefix(colspan).reduce(0, +)

[jrturton]

nice, I prefer that a lot!

[QuietMisdreavus]

This looks great, thanks for posting the fix!

[QuietMisdreavus]

@swift-ci Please test