Language agnostic schema

Once #84 is done we're getting close to the codegen input being language agnostic. It "just" needs to be a form other than Rust and to only have attributes that make sense beyond Rust. Strawman to incite debate: use https://toml.io/en/, it's simple, widely supported, supports comments, and more than sufficient to capture what we need.

*Today*

```rust
/// [COLR (Color)](https://learn.microsoft.com/en-us/typography/opentype/spec/colr#colr-header) table
table Colr {
/// Table version number - set to 0 or 1.
#[version]
version: u16,
/// Number of BaseGlyph records; may be 0 in a version 1 table.
num_base_glyph_records: u16,
/// Offset to baseGlyphRecords array (may be NULL).
#[nullable]
#[read_offset_with($num_base_glyph_records)]
base_glyph_records_offset: Offset32<[BaseGlyph]>,
```

*Tomorrow*

```toml
tag = "COLR"
root = "Colr"

[table.Colr]
comment = "[COLR (Color)](https://learn.microsoft.com/en-us/typography/opentype/spec/colr#colr-header) table"

[table.Colr.version]
type = "u16"
attrib = ["version"]
comment = "Table version number - set to 0 or 1."

[table.Colr.num_base_glyph_records]
type = "u16"
comment = "Number of BaseGlyph records; may be 0 in a version 1 table."

[table.Colr.base_glyph_records_offset]
type = "Offset32"
attrib = ["nullable"]
comment = "Offset to baseGlyphRecords array (may be NULL)."
```

Start by reviewing issue #84 and the current Rust-form schema shown here, then evaluate the proposed TOML representation against the codegen input needs. Done should be a decided language-agnostic schema, including which attributes remain valid beyond Rust, with agreement on how it will replace the current form.

Written by the indexing model from the issue text.