Clean up typography API output

[jamesmockett]

Adding tests for Source Foundation's token and typography output has highlighted that the output from the typography API could be cleaned up without affecting functionality.

Currently it looks like this:

font-family: "GT Guardian Titlepiece", Georgia, serif;
font-size: 4.375rem;
line-height: 1.15;
font-weight: 700;
;

/*
 * Child elements (e.g. <Link/>) can use this variable
 * to set the thickness of their underline.
 *
 * The thickness for each font type and weight is defined
 * in the underlineThickness object.
 */
--source-text-decoration-thickness: 6px;

The comment is unnecessary and the output includes additional whitespace and a spurious semicolon.

In addition, the object style output includes a fontStyle property which is always set to undefined.

(A more extensive refactoring of the typography code is covered by #1039)