[glypsh-reader] fix ParseIntError when custom param has unquoted string

[anthrotype]

If you create a New Font from Glyphs.app and then add a 'Variable Font Origin' custom parameter (which we currently do support) and set this to the first and only master that the newly created font comes with (which Glyphs.app assigns the id of m01 using an unquoted plist string), and then attempt to compile, you will get a ParseIntError when parsing the custom parameters, because glyphs-reader thinks that it is an integer.

I add a reproducer first, and follow up with the actual fix.

[anthrotype]

https://github.com/googlefonts/fontc/actions/runs/7104626540/job/19340026073?pr=626

failures:

---- font::tests::vf_origin_unquoted_string stdout ----
thread 'font::tests::vf_origin_unquoted_string' panicked at glyphs-reader/src/font.rs:411:84:
called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    font::tests::vf_origin_unquoted_string

test result: FAILED. 50 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.08s