Add label name for ss01-ss20 (and other info for cv01-cv99 if not cost too much)

[CoelacanthusHex]

Is your feature request related to a problem? Please describe.

Can't know which ssXX and cvXX means without reading the document.

Describe the solution you'd like

The OpenType spec allows the font developers to specify a label name for ss01-ss20 via the Feature Parameters table, and to specify a label name and other information for cv01-cv99.

https://learn.microsoft.com/en-us/typography/opentype/spec/features_pt#ssxx

https://learn.microsoft.com/en-us/typography/opentype/spec/features_ae#cv01-cv99

Some fonts like Monaspace have used the former to describe the meaning of ssXX in font files.

Describe alternatives you've considered

Seems no other way.

Additional context

Example of Monaspace.

>>> hb-info --list-features MonaspaceArgonVarVF\[wght,wdth,slnt\].ttf                                                                
Layout features information:

Showing all font features with duplicates removed.

Table: GSUB
    Feature: aalt
    Feature: calt
    Feature: case
    Feature: ccmp
    Feature: cv30
    Feature: cv60
    Feature: cv61
    Feature: dnom
    Feature: fina
    Feature: frac
    Feature: init
    Feature: ital
    Feature: liga
    Feature: locl
    Feature: medi
    Feature: numr
    Feature: ordn
    Feature: sinf
    Feature: ss01   SS01: Equal Symbols
    Feature: ss02   SS02: Comparisons
    Feature: ss03   SS03: Arrows
    Feature: ss04   SS04: HTML Tags
    Feature: ss05   SS05: F# Shapes
    Feature: ss06   SS06: Markdown Strings
    Feature: ss07   SS07: Centered Colon
    Feature: ss08   SS08: Centered Period
    Feature: ss09   SS09: Double Arrows
    Feature: subs
    Feature: sups
Table: GPOS