Use fields from Typst 0.7.0 when possible

[PgBiel]

Should help make calculations accurate, and even improve performance (we don't have to measure twice to find the value of negative em)

Tasks:

• [x] Use fields to convert lengths to pt
• [x] Use fields to work with stroke properties
• [x] Other possibilities? Maybe is-infinite-len, but it's ok as is for now.