Reland PR #7 manually

Credit goes to Ruben (ruben-hude). While doing this, Leedehai fixed
a few issues in the original PR:
(1) out-of-sync manual
(2) subscripts and superscripts remain bold in vectorarrow()

physica-manual.typ

@@ -164,16 +164,16 @@ All symbols need to be used in *math mode* `$...$`.
   [`vb(a),vb(mu_1)` #sym.arrow $vb(a),vb(mu_1)$],
   [vector, bold],
 
-  [`vectorarrow(`_content_`)`],
-  [`va`],
-  [`va(a),va(mu_1)` #sym.arrow $va(a),va(mu_1)$],
-  [vector, arrow],
-
   [`vectorunit(`_content_`)`],
   [`vu`],
   [`vu(a),vu(mu_1)` #sym.arrow $vu(a),vu(mu_1)$],
   [unit vector],
 
+  [`vectorarrow(`_content_`)`],
+  [`va`],
+  [`va(a),va(mu_1)` #sym.arrow $va(a),va(mu_1)$],
+  [vector, arrow \ #sub[(not bold: see ISO 80000-2:2019)]],
+
   [`gradient`],
   [`grad`],
   [`grad f` #sym.arrow $grad f$],

physica.typ

@@ -155,28 +155,36 @@
 #let vectorbold(a) = $bold(italic(#a))$
 #let vb = vectorbold
 
-#let __vectoraccent(a, accent) = {
-  let bold_italic(e) = math.bold(math.italic(e))
+#let __vectoraccent(a, accent, be_bold) = {
+  let maybe_bold(e) = if be_bold {
+    math.bold(math.italic(e))
+  } else {
+    math.italic(e)
+  }
   if type(a) == "content" and a.func() == math.attach {
     math.attach(
-      math.accent(bold_italic(a.base), accent),
-      t: if a.has("t") { math.bold(a.t) } else { none },
-      b: if a.has("b") { math.bold(a.b) } else { none },
-      tl: if a.has("tl") { math.bold(a.tl) } else { none },
-      bl: if a.has("bl") { math.bold(a.bl) } else { none },
-      tr: if a.has("tr") { math.bold(a.tr) } else { none },
-      br: if a.has("br") { math.bold(a.br) } else { none },
+      math.accent(maybe_bold(a.base), accent),
+      t: if a.has("t") { maybe_bold(a.t) } else { none },
+      b: if a.has("b") { maybe_bold(a.b) } else { none },
+      tl: if a.has("tl") { maybe_bold(a.tl) } else { none },
+      bl: if a.has("bl") { maybe_bold(a.bl) } else { none },
+      tr: if a.has("tr") { maybe_bold(a.tr) } else { none },
+      br: if a.has("br") { maybe_bold(a.br) } else { none },
     )
   } else {
-    math.accent(bold_italic(a), accent)
+    math.accent(maybe_bold(a), accent)
   }
 }
-#let vectorarrow(a) = __vectoraccent(a, math.arrow)
-#let va = vectorarrow
 
-#let vectorunit(a) = __vectoraccent(a, math.hat)
+#let vectorunit(a) = __vectoraccent(a, math.hat, true)
 #let vu = vectorunit
 
+// According to "ISO 80000-2:2019 Quantities and units — Part 2: Mathematics"
+// the vector notation should be either bold italic or non-bold italic accented
+// by a right arrow
+#let vectorarrow(a) = __vectoraccent(a, math.arrow, false)
+#let va = vectorarrow
+
 #let gradient = $bold(nabla)$
 #let grad = gradient