Change `PixelsPerInch` to a class with `static` properties (issue 14579)
Change `PixelsPerInch` to a class with `static` properties (issue 14579)

*Please note:* I'm completely fine with this patch being rejected, and the issue instead closed as WONTFIX, since this is unfortunately a case where the TypeScript definitions dictate how we can/cannot write JavaScript code.

Apparently the TypeScript definitions generation converts the existing `PixelsPerInch` code into a `namespace` and simply ignores the getter; please see https://github.com/mozilla/pdfjs-dist/blob/a7fc0d33a11d032741f44d0b623780253747da10/types/src/display/display_utils.d.ts#L223-L226

Initially I tried tagging `PixelsPerInch` as en `@enum`, see https://jsdoc.app/tags-enum.html, however that unfortunately didn't help.
Hence the only good/simple solution, as far as I'm concerned, is to convert `PixelsPerInch` into a class with `static` properties. This patch results in the following diff, for the `gulp types` build target:
```diff
@@ -195,9 +195,10 @@
      */
     static toDateObject(input: string): Date | null;
 }
-export namespace PixelsPerInch {
-    const CSS: number;
-    const PDF: number;
+export class PixelsPerInch {
+    static CSS: number;
+    static PDF: number;
+    static PDF_TO_CSS_UNITS: number;
 }
 declare const RenderingCancelledException_base: any;
 export class RenderingCancelledException extends RenderingCancelledException_base {
```

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