Fix fallback font chain not persistence when specified weight

[HelloYeew]

Now if the requested font with that weight is not available it will start looking in fallback chain. This also applied in the icon usage too since our icon usage built on top of font renderer. So if you render mix of normal text and icon (using char.ConvertFromUtf32) with "bold" weight, in the icon part it will finding the "MaterialIcon-Bold" that we don't have since we only have regular, this PR make the fallback more weight-tolerant by make it look for regular weight instead too.

Also since I use mix of icon and text in SpriteText so I add a new extension for IconUsage for make my sanity better in char.ConvertFromUtf32 everywhere.

// before
Text = $"Play {char.ConvertFromUtf32((int)IconUsage.PlayArrow))}
// new
Text = $"Play {IconUsage.PlayArrow.ToGlyph()}