<Assignment> not recognized when followed by a integer

[ArturJD96]

Hello! Thank you for all this amazing stuff around!

I have identified a misbehaviour of lexer. When assignment is proceeded by a numeric, it is ignored by the lexer and is not reflected in the music document.

from ly.document import Document
from ly.music import document

str_INCORRECT_result = """
\\version "2.24.3"
\\include "def.ly"
numeric = 2 % this line messes up the lexer
assignment = \\relative c {
    a4
}
"""

str_correct_result = """
\\version "2.24.3"
\\include "def.ly"
numeric = "2"
assignment = \\relative c {
    a4
}
"""

d_correct = document(Document(str_correct_result))
d_INCORRECT = document(Document(str_INCORRECT_result))

print(d_correct.dump())
# <Document>
#   <Version '\\version'>
#     <String '"'>
#   <Include '\\include'>
#     <String '"'>
#   <Assignment 'numeric'>
#     <String '"'>
#   <Assignment 'assignment'> <-- CORRECT
#     <Relative '\\relative'>
#       <Note 'c'>
#       <MusicList '{'>
#         <Note 'a'>
#           <Duration '4'>

print(d_INCORRECT.dump())
# <Document>
#   <Version '\\version'>
#     <String '"'>
#   <Include '\\include'>
#     <String '"'>
#   <Assignment 'numeric'>
#     <Unpitched>
#       <Duration '2'>
#   <Relative '\\relative'> <-- NOT CORRECT – <Assignment> MISSING!
#     <Note 'c'>
#     <MusicList '{'>
#       <Note 'a'>
#         <Duration '4'>