utilities: simplify and fix conversion of relative lengths to pt

The `convert-length-to-pt` previously used regex to retrieve the ratio
portion of the relative length. However, this is not necessary as
relative lengths expose their components through their `ratio` and
`length` fields. We can simply pass each component to the respective
conversion function for their type and add the results.

The old implementation also had an issue when the `length` component is
compound and positive (e.g., 1em + 1pt). It would throw an error because
Typst does not allow comparing positive compound lengths with 0pt. For
some reason, no error would be thrown for negative compound lengths (not
sure why), but the result of the conversion appears to become 0pt.

src/utilities.typ

@@ -197,37 +197,10 @@
     } else if type(len) == _fraction_type {
         convert-fraction-type-topt(len, frac_amount, frac_total)
     } else if type(len) == _rel_len_type {
-        if styles == none {
-            panic("Cannot convert relative length to pt ('styles' not specified).")
-        }
-
-        let ratio_regex = regex("^\\d+%")
-        let ratio = repr(len).find(ratio_regex)
-
-        if ratio == none {  // 2em + 5pt  (doesn't contain 100% or something)
-            measure(line(length: len), styles).width
-        } else {  // 100% + 2em + 5pt  --> extract the "100%" part
-            if page_size == none {
-                panic("Cannot convert relative length to pt ('page_size' not specified).")
-            }
-
-            // SAFETY: guaranteed to be a ratio by regex
-            let ratio_part = eval(ratio)
-            assert(type(ratio_part) == _ratio_type, message: "Eval didn't return a ratio")
-
-            let other_part = len - ratio_part  // get the (2em + 5pt) part
+        let ratio_part_pt = convert-ratio-type-to-pt(len.ratio, page_size)
+        let length_part_pt = convert-length-type-to-pt(len.length, styles)
 
-            let ratio_part_pt = if is-infinite-len(page_size) { 0pt } else { ((ratio_part / 1%) / 100) * page_size }
-            let other_part_pt = 0pt
-
-            if other_part < 0pt {
-                other_part_pt = -measure(line(length: -other_part), styles).width
-            } else {
-                other_part_pt = measure(line(length: other_part), styles).width
-            }
-
-            ratio_part_pt + other_part_pt + 0pt
-        }
+        ratio_part_pt + length_part_pt
     } else {
         panic("Cannot convert '" + type(len) + "' to length.")
     }