Add \rhythm markup command
\rhythm takes a music expression and typesets it using an embedded score, which is mostly useful for tempo specifications. This feature has asked about frequently (last time today on -user-fr), and is found in the popular LSR snippets https://lsr.di.unimi.it/LSR/Item?id=204 and https://lsr.di.unimi.it/LSR/Item?id=1029
Its internals are powered by new context types, StandaloneRhythmic(Score|Staff|Voice). One reason is to allow global tweaks with \layout { \context { \StandaloneRhythmicXXX ... } }. Another is to avoid settings from stylesheets made with \context { \Voice ... } being applied to \rhythm accidentally, as they're likely not intended there. They can be applied to StandaloneRhythmicVoice explicitly if needed.
Known issue: \rhythm { 8 } does not work because 8 is interpreted as a duration. One has to use \rhythm { { 8 } }.
Fixes #6221 (closed)