processing :: in the meta-data

AMR files usually start with an id and the sentence before the actual PENMAN graph comes

```
# ::id any-ID-001.1
# ::snt the cat is sleeping
# ::save-date Sat Jul 20, 2024 ::file test_0001_2.txt
( s / cleep-01
:ARG0 ( c / cat))
```

the penman lib parses this without any problem and provides it in the `metadata` dictionary. Multiple ::keys are parsed correctly

However I cam across sentences which contain `::`

```
# ::snt this must be separated using :: unless it is a single line
...
```
unfortunately penman-lib cuts the sentence at the `::` and creates a metadata-entry with a space as key.
For other comment lines having mulitple keys is OK, but for the line containing `::snt` is forbids having sentences with `::`. Could this be changed?

Start at the metadata parsing entry point that populates the metadata dictionary and inspect how :: delimiters are handled in ::snt lines. Reproduce the example with an embedded ::, then adjust the parsing behavior so the sentence remains intact while other metadata keys continue to parse correctly; add or update a regression test if the repository's parser tests cover this behavior.

Written by the indexing model from the issue text.