Add string alignment handling to FormatWith functionality

[TheodoreBrinkman]

Additional values & tests for a left-padding format string.

This type of format string does not currently work in FormatWith.
I suspect a format-string parse issue, where it assumes the first format-string delimiter is ':', rather than a potential ','.
I've been trying to find workarounds, but haven't managed to do so yet.

[TheodoreBrinkman]

FormatWith/Internal/FormatHelpers.cs around line 48/138, the parser checks for the first instance of ':', but it is possible for the overall format string to begin with ',' followed by a number indicating alignment then ':' and the rest of the format string. the ',' plus number denotes padding the value with spaces.
A positive value indicates left-padding.
A negative value indicates right-padding.

It might be worth switching to a RegEx parser for the format string, but I'm not sure how complex that would be.
Otherwise, the fix would involve finding the ':' if it's there, then looking for a preceeding ',' to parse out the number, and making sure you build the format value (defined on line 47/137) to include the ',alignment:formatString' combination as necessary.

Note: Line references appear to be 40/39, and 109/108 respectively, in the 4.0 branch.

[TheodoreBrinkman]

Here's a .NET Fiddle example of the format string with alignment in action.

.Net Fiddle - Format String with Alignment

[TheodoreBrinkman]

A quick .NET Fiddle sample comparing a regex parse to a IndexOf/Substring parse.
https://dotnetfiddle.net/tWPTyJ

[TheodoreBrinkman]

@crozone I'm not familiar with the innards of the library. Can you let me know where else I might need to make changes to handle the alignment portion of a format string? I'm thinking it might be useful to provide the parsing functionality as a function that returns an object so it can be used by handlers, but I'm not certain.

[TheodoreBrinkman]

Also, if you're willing to have a breaking change for 4.0, you could re-structure the special handler designation such that it can have alignment, handler, and formatting, and provide a way to register multiple handlers, rather than requiring library users to write a single method to handle any and all special handlers they want to use.

For example: {token[,alignment][;handler][:formatString]}
{someDate,15;uppercase:yyyy MMM dd} to convert a date to 1900 JAN 24, and left-pad it out to 15 characters.

Looking at the .NET composite formatting page, this would be compatible with the existing {index[,alignment][:formatString]} notation.

[TheodoreBrinkman]

Passes all existing tests and newly created tests.

[TheodoreBrinkman]

I've also got a set of changes for the 4.0 branch, but I've always been in TFS shops, so I'm having issues figuring out the process for Git in Visual Studio. And it'll probably need a look at whether I've missed anything regarding the use of ReadOnlySpan rather than String.