Fix `remove_compound_assignment` rule to avoid copying variable tokens

[jeparlefrancais]

Closes #174

• [x] add entry to the changelog

[github-actions[bot]]

Coverage after merging fix-remove-compound-assignment-copy-tokens into main will be

86.92%

Coverage Report for Changed Files

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src/rules
remove_comments.rs	70.91%	100%	67.21%	71.96%	159–161, 165–170, 172, 174, 176–178, 180–182, 184–186, 188–190, 192–194, 196–198, 200–202, 204–206, 208–210, 212–214, 216–218, 220–222, 224–226, 228–230, 77–79
remove_compound_assign.rs	95.90%	100%	89.29%	96.76%	272–274, 55, 83
remove_spaces.rs	70.55%	100%	67.21%	71.50%	159–161, 165–170, 172, 174, 176–178, 180–182, 184–186, 188–190, 192–194, 196–198, 200–202, 204–206, 208–210, 212–214, 216–218, 220–222, 224–226, 228–230, 249, 77–79