Improve SVG generation logic

[t-hamano]

The issue was originally filed in the Gutenberg repository.

WordPress/gutenberg#60188

There is currently a visual difference between the "text" preview displayed in the Gutenberg project's font library modal and the "svg" preview generated by this repository. This seems to be due to the logic for generating the text itself being different between the two (see this comment)

Personally, I think the "text" preview in the Gutenberg repository makes more sense. That is, the format is as follows:

{FontName} {FontWeightName}

or

{FontName} {FontWeightName} {FontStyleName}