add an option to opt out of USE_TYPO_METRICS

[kseino]

Added useTypoMetrics options to set REGULAR to fsSelection.
https://docs.microsoft.com/en-us/typography/opentype/spec/os2#fsselection

Use cases
We use custom icon fonts generated thanks to svg2ttf in Flutter project.
Flutter renders extra ascent when Use Typo Metrics is enabled. useTypoMetrics options will solve this problem.

#95 (comment)

[kseino]

@puzrin
Could you review please 🙏

[puzrin]

Could you use fork for a while? I'm still busy with extremely urgent fixes on my project, which made all this stuff possible.

Sorry for unexpected delays.

[kseino]

related
flutter/flutter#98909

[kseino]

@puzrin
Could you please review this PR?

We are looking forward to this option!

[velkrissz]

I encountered the same problem when generating an icon font for a Flutter app.

I avoided the issue without changing the Use Typo Metrics flag. (Tested on Android and iOS) Based on some experimenting, the problem disappears if the OS/2 sTypoLineGap is set to 0. Based on my understanding, this won't cause any problem as we use this for icons, not real fonts,
I monkey-patched the Font "class" using the following snippet based on the actual implementation and it seems to be working just fine.

import * as sfnt from "svg2ttf/lib/sfnt.js";
// ...
Object.defineProperty(sfnt.Font.prototype, "int_lineGap", { value: 0 });

This bypasses the built-in 9% of font ascent minus descent calculation.
I suggest adding a lineGap parameter to the options object. (SVG font face does not define a similar param.)