fix(gatsby-transformer-remark): don't convert Date objects

[stefanprobst]

gatsby-transformer-remark parses frontmatter with gray-matter and then iterates through frontmatter fields again to transform Dates into strings.

This PR proposes to only allow types from the YAML 1.2 spec, which has no timestamp type anymore (the 1.1 spec has). This way, dates will automatically be parsed as strings.

For the different schema options that the yaml parser accepts see here.

[pieh]

I actually wonder if this is still problem (fact that Date object are potentially used). I remember fixing inconsistencies when Date object was used, then cached in .cache/redux-state.json and read from in next run - it might be just unnecessary code and we don't need to specifically declare schema without that don't support Date objects here?

[stefanprobst]

Ah indeed, just checked and it works fine because of isMixOfDateObjectsAndDateStrings.

[freiksenet]

@stefanprobst @pieh Is this PR still relevant?

[pieh]

Yeah, it's still worth to get in, will just validate that change now so at least 2 sets of eyes checked it.

[pieh]

Validated and works as expected. Thanks @stefanprobst!

[zslabs]

I have a date field within my project that has some written as strings, and others not (have content creators doing both unfortunately); and this PR appears to be breaking the sorting functionality I'm using.