Add more specific grid support detection

[andrew-nowak]

Product of bbc/simorgh#2063

Is your feature request related to a problem? Please describe.
@supports (display:grid) doesn't ensure that the browser correctly supports grid. In bbc/simorgh#2063 we saw that a Firefox 48 fork had enabled its CSS grid feature flag, but the CSS Grid implementation in Firefox 48 was buggy. We should use a later Grid feature as a way of attempting to ensure correct grid performance.

Describe the solution you'd like
An example found during debugging in bbc/simorgh#2063 was @supports (grid-template-columns: fit-content(300px) fit-content(300px) 1fr), as the fit-content css function was added in a later version of Firefox. As part of this issue, you should ensure that this both causes the KaiOS browser to use the grid fallback, and does not disable grid on browsers that correctly support grid. If it is not sufficient, please investigate alternatives.

This can then be exported from psammead, and used in place of @supports (display: grid) where used in psammead and simorgh, eg. @supports (${supportsGrid}).

Ensure that all grid declarations in simorgh and psammead are nested within this new @supports rule (this may be part of a future issue.)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Testing notes
[Tester to complete]

Dev insight: Will Cypress tests be required or are unit tests sufficient? Will there be any potential regression? etc

Additional context
Add any other context or screenshots about the feature request here.

[andrew-nowak]

superceded by #1364