Previously, `DirectWrite` had been following the text system
implementation on `macOS`, using the font's postscript name to
differentiate between different font faces. However, I noticed
occasional rendering issues, such as fonts incorrectly rendering as
italics. Later, I discovered that on `Windows`, the postscript name is
**not** unique. Surprisingly, even the same font can have different
postscript names on macOS and Windows! It's hard to believe! The
postscript name of a font face should be obtained from the same font
table. Why would the same font face have different names?

For example, the postscript name of ZedMono on `macOS` is
`Zed-Mono-Bold-Extended-Italic`, while on `Windows`, it is
`Zed-Mono-Extended`, missing weight and style information, leading to
incorrect rendering.

This PR introduces a `FontIdentifier` struct to uniquely identify font
faces.

Release Notes:

- N/A