Add direct sub-example ID references

[fmatter]

This PR adds the ability to assign custom IDs to individual sub-examples and reference them directly, without manually specifying the letter suffix.

Syntax

Place {#id} in the header line of an interlinear example:

::: {#ex:example1 .ex}
a.
| {#ex:dutch} Dutch (Germanic)
| Deze zin is in het nederlands.
| DEM sentence AUX in DET dutch.
| This sentence is dutch.

b.
| {#ex:mapudungun} Mapudungun (Isolate)
| küpatueyew chi ḻuan
| come-APPL-INV-IND-3-3ACT DEF guanaco
| 'The guanaco came to him.'
:::

The ID marker is removed from the output, leaving clean language headers.

References

Direct references to sub-examples:

• [@ex:dutch] → (1a)
• [@ex:mapudungun] → (1b)

The reference format uses no space between the number and letter, matching the parent example's automatic numbering.

Backwards compatibility

The old manual suffix syntax continues to work:

• [@ex:example1 a] → (1 a)
• [@last b] → (1 b)

Both methods can coexist in the same document.

Benefits

• Stable references: Sub-example IDs don't break when reordering
• Empty headers: Works with | {#id} for language-consistent examples
• HTML anchors: Custom IDs are available as direct links in HTML output