Add support for LilyPond syntax (#5117)

* Add support for LilyPond syntax

See: https://lilypond.org/

* Fix alpha order of LilyPond

* Add support for custom executable and options to lilypond linter

Enhances the lilypond linter with configurable options:
- Add g:ale_lilypond_lilypond_executable for custom lilypond binary path
- Add g:ale_lilypond_lilypond_options for additional command-line flags
- Refactor linter to use GetCommand() function for dynamic command building
- Add linter tests covering configuration scenarios
- Update documentation with usage examples and proper formatting

---------

Co-authored-by: samb0t <sambottoni@gmail.com>

test/handler/test_lilypond.vader

@@ -0,0 +1,49 @@
+Before:
+  runtime ale_linters/lilypond/lilypond.vim
+  call ale#linter#Reset()
+
+Execute(The lilypond handler should parse a single error correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 2,
+  \     'col': 5,
+  \     'type': 'E',
+  \     'text': 'syntax error, unexpected NOT_A_TOKEN',
+  \   }
+  \ ],
+  \ ale_linters#lilypond#lilypond#Handle(0, [
+  \   'test.ly:2:5: error: syntax error, unexpected NOT_A_TOKEN'
+  \ ])
+
+Execute(The lilypond handler should parse warnings and errors):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 3,
+  \     'col': 1,
+  \     'type': 'W',
+  \     'text': 'deprecated syntax',
+  \   },
+  \   {
+  \     'lnum': 5,
+  \     'col': 12,
+  \     'type': 'E',
+  \     'text': 'unknown symbol',
+  \   }
+  \ ],
+  \ ale_linters#lilypond#lilypond#Handle(0, [
+  \   'test.ly:3:1: warning: deprecated syntax',
+  \   'test.ly:5:12: error: unknown symbol'
+  \ ])
+
+Execute(The lilypond handler should ignore non-matching lines):
+  AssertEqual
+  \ [],
+  \ ale_linters#lilypond#lilypond#Handle(0, [
+  \   'This is some unrelated output',
+  \   'Another line without structure'
+  \ ])
+
+After:
+  call ale#linter#Reset()

test/linter/test_lilypond.vader

@@ -0,0 +1,35 @@
+Before:
+  call ale#assert#SetUpLinterTest('lilypond', 'lilypond')
+  call ale#test#SetFilename('test.ly')
+
+  let g:ale_lilypond_lilypond_executable = 'lilypond'
+  let g:ale_lilypond_lilypond_options = ''
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The executable should default to lilypond):
+  AssertLinter 'lilypond',
+  \ ale#Escape('lilypond') . ' --loglevel=WARNING -dbackend=null -dno-print-pages -o /tmp %t 2>&1'
+
+Execute(Should be able to set a custom executable):
+  let g:ale_lilypond_lilypond_executable = 'bin/lilypond'
+
+  AssertLinter 'bin/lilypond',
+  \ ale#Escape('bin/lilypond') . ' --loglevel=WARNING -dbackend=null -dno-print-pages -o /tmp %t 2>&1'
+
+Execute(Should be able to set custom options):
+  let g:ale_lilypond_lilypond_options = '--include=/path/to/includes'
+
+  AssertLinter 'lilypond',
+  \ ale#Escape('lilypond') . ' --loglevel=WARNING -dbackend=null -dno-print-pages -o /tmp'
+  \ . ' --include=/path/to/includes %t 2>&1'
+
+Execute(Should be able to set both custom executable and options):
+  let g:ale_lilypond_lilypond_executable = '/usr/local/bin/lilypond'
+  let g:ale_lilypond_lilypond_options = '--include=/custom/path -dlog-file=/tmp/lily.log'
+
+  AssertLinter '/usr/local/bin/lilypond',
+  \ ale#Escape('/usr/local/bin/lilypond') . ' --loglevel=WARNING -dbackend=null -dno-print-pages -o /tmp'
+  \ . ' --include=/custom/path -dlog-file=/tmp/lily.log %t 2>&1'
+