Add (Letter Character Face Ratio) [PADG](?) axis

### Requirements

By ticking the cases (or putting x between the square brackets in text mode), you confirm the following:

- [x] I have inspected the current [Axis Registry](https://github.com/googlefonts/axisregistry/tree/main/Lib/axisregistry/data) and there is not a registered custom axis that could be used for the variation purpose on the upcoming font project.
- [x] The metadata fields of the proposed axis meet the *Axis Requirements* as specified in the Protocol (linked above).

### Font project(s) using the axis

[https://github.com/chiron-fonts/chiron-hei-hk](https://github.com/chiron-fonts/chiron-hei-hk)

### Short description of what the axis does

Enables a font to vary the letter/character face ratio (字面率) for a CJK font to provide more spacing between characters.

### Image

The glyph on the left has a ratio of 92%, the one on the right has a ratio of 95%.

### Why is the axis needed

When the point size becomes smaller (or larger), there is a perceived change in the space between ideographs. To address situations where (a) tracking cannot be changed, or (b) where changing tracking is not tenable (especially in mixed script scenarios), CJK foundries change the letterspace ratio in order to reduce the text density and perception of airiness.

This axis is discernibly different than `SPAC` as it does not cause reflow / affect horizontal advance of the line of text. It has some similarity with `opsz` conceptually as it is related to improving rendering at smaller sizes, but I believe the intention is sufficient different to warrant its own axis, especially as it is independent of other optical corrections one might implement.

My suggestion is that `PADG` should be a % reduction from 100%. So `-10` would represent a 10% reduction in the character face ratio.

***Note*** The `PADG` naming convention is one that @tamcy (of Chiron Hei) came up with (referencing "Padding"), and is open to changing. Another option that could work is `VBDY`, referencing the Japanese term "Virtual Body").

### Axis metadata fields

```
#(PADG) based on (https://github.com/chiron-fonts/chiron-hei-hk)
tag: "PADG"
display_name: "Letter Character Face Ratio"
min_value: -100.0
default_value: 0.0
max_value: 100.0
precision: 1
fallback {
name: "Default"
value: 0.00
}
fallback_only: false
description: "Adjust the letter character face ratio of the ideographic characters."
````