Issue 3487: Make several special characters with or without backslash "shorthands"

Issue 3487: Make several special characters with or without backslash "shorthands"

Single non-alphanumeric ASCII characters not requiring special
treatment in lexer or parser can now be redefined like escaped
identifiers.  The same holds for escaped non-alphanumeric ASCII
characters.  The identifying name you use for redefining them is the
string corresponding to the full shorthand, in contrast to escaped
identifiers where the identifying name omits the initial backslash.

Notable shorthands not treated specially in the parser (some of them
newly so) can be seen in the following definitions from
scm/declarations-init.ly:

"|" = #(make-music 'BarCheck)
"[" = #(make-span-event 'BeamEvent START)
"]" = #(make-span-event 'BeamEvent STOP)
"~" = #(make-music 'TieEvent)
"(" = #(make-span-event 'SlurEvent START)
")" = #(make-span-event 'SlurEvent STOP)
"\\!" = #(make-span-event 'CrescendoEvent STOP)
"\\(" = #(make-span-event 'PhrasingSlurEvent START)
"\\)" = #(make-span-event 'PhrasingSlurEvent STOP)
"\\>" = #(make-span-event 'DecrescendoEvent START)
"\\<" = #(make-span-event 'CrescendoEvent START)
"\\[" = #(make-span-event 'LigatureEvent START)
"\\]" = #(make-span-event 'LigatureEvent STOP)
"\\~" = #(make-music 'PesOrFlexaEvent)
"\\\\" = #(make-music 'VoiceSeparator)