Add Speed [ SPED ] axis

### Requirements

- [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

Playwrite (VF parent font)

### Short description of what the axis does

As the speed increase, the joins and ending strokes go from slow (0%), with more rounded arches, to fast (100%), making the junctions more diagonal and fluid.

### Image


### Why is the axis needed

Speed is a decisive factor in the construction of the letters, which alters their shape and, therefore, the rhythm and fluidity of the writing. This axis would offer specific control over that factor, allowing a fine-tuning of those features.

### Axis metadata fields

```
# SPED based on github.com/TypeTogether/Playwrite
tag: "SPED"
display_name: "Speed"
min_value: 0
default_value: 0
max_value: 100
precision:
fallback {
name: "Default"
value: 0.00
}
fallback_only: false
description: "As the speed increase, the joins and ending strokes go from slow (0%),"
" with more rounded arches to fast (100%), making the junctions more diagonal and fluid."
````