Added vowel transitions for lyrics

input/regression/vowel-transition-padding-broken.ly

+\version "2.21.0"
+\header
+{
+
+  texidoc = "For vowel transitions, left/right padding are
+independent of left-broken/right-broken padding."
+
+}
+
+\layout {
+  ragged-right = ##t
+  indent = #0
+}
+
+music = {
+  r2 a ~ \break
+  a2 a \bar "||" \break
+}
+
+words = \lyricmode {
+  ah \vowelTransition
+  oh
+}
+
+<<
+  \new Voice = "A"  {
+    \music
+    \music
+    \music
+    r2 a \break
+    a2 r2 \bar "|."
+  }
+  \new Lyrics \lyricsto "A" {
+    \override VowelTransition.minimum-length = #4
+    \words
+    \once \override VowelTransition.bound-details.left.padding = #2
+    \once \override VowelTransition.bound-details.right.padding = #4
+    \words
+    \once \override VowelTransition.bound-details.right-broken.padding = #2
+    \once \override VowelTransition.bound-details.left-broken.padding = #4
+    \words
+    \once \override VowelTransition.after-line-breaking = ##t
+    \once \override VowelTransition.bound-details.left.padding = #2
+    \once \override VowelTransition.bound-details.right-broken.padding = #2
+    \once \override VowelTransition.bound-details.left-broken.padding = #4
+    \once \override VowelTransition.bound-details.right.padding = #4
+    \words
+  }
+>>