consolidate PackageES versioning in /custom.props

[DHowett-MSFT]

This location and name is practically mandated by PackageES. Sorry ☹️.

This will ensure that all artifacts that we produce are versioned properly:

thing	version (example)
exe, dll, .net assembly	0.7.1911.22009
nupkg	0.7.191122009
appx	0.7.3269.0

For reference, here's the version format:

EXE, DLL, .NET Assembly

0.7.1911.22009
^ ^  ^ ^  ^  ^
| |  | |  |  `-Build # on that date
| |  | |  `-Day
| |  | `-Month
| |  `-Year
| `-Minor
`-Major

NuGet Package

0.7.191122009
^ ^  ^ ^ ^  ^
| |  | | |  `-Build # on that date
| |  | | `-Day
| |  | `-Month
| |  `-Year
| `-Minor
`-Major

AppX Package

0.7.03269.0
^ ^ ^  ^^ ^
| | |  || `-Contractually always zero (a waste)
| | |  |`-Build # on that date
| | |  `-Number of days in [base year]
| | `-Number of years since [base year]
| `-Minor
`-Major

[base year] = $(XesBaseYearForStoreVersion)

It is expected that the base year is changed every time
the version number is changed.

[zadjii-msft]

I'm okay with this, but "y tho"

[zadjii-msft]

Does it have to be named custom.props? That doesn't seem particularly specific to this scenario

[DHowett-MSFT]

Unfortunately, it does.