Refactor subgrid workarounds once it's supported in necessary browsers

[adamkudrna]

Once supported:

• form fields in FormLayout can use subgrid instead of display: contents with two mandatory divs,
• CheckboxField and Toggle can reflect labelPosition="before" inside FormLayout.

[adamkudrna]

UPDATE September 2023: subgrid is now supported in all major browsers including Chrome joining as last in August's version 117. However, there is some fieldset/legend magic that prevents subgrid from being used on <fieldset> element (as per the spec (sic!), the magic adds an additional virtual box which breaks the subgrid).

As a result, subgrid cannot be used on <fieldset> elements until the following Chrome issue is resolved:

https://bugs.chromium.org/p/chromium/issues/detail?id=1473242