Unexpected error while parsing `\boldsymbol`

```julia
julia> generate_tex_elements(L"\boldsymbol{\nabla}")
ERROR: TeXParseError: unexpected error while parsing
at position 12 (string index 12)
\boldsymbol{\nabla}
^
Stack before
[1] TeXExpr :expr

Stacktrace:
[1] texparse(data::String; showdebug::Bool)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/parser/parser.jl:303
[2] texparse
@ ~/.julia/packages/MathTeXEngine/na9MW/src/parser/parser.jl:287 [inlined]
[3] _broadcast_getindex_evalf
@ ./broadcast.jl:683 [inlined]
[4] _broadcast_getindex
@ ./broadcast.jl:656 [inlined]
[5] _getindex
@ ./broadcast.jl:679 [inlined]
[6] _broadcast_getindex
@ ./broadcast.jl:655 [inlined]
[7] getindex
@ ./broadcast.jl:610 [inlined]
[8] copy
@ ./broadcast.jl:912 [inlined]
[9] materialize
@ ./broadcast.jl:873 [inlined]
[10] generate_tex_elements(str::LaTeXStrings.LaTeXString, font_family::MathTeXEngine.FontFamily)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/engine/layout.jl:301
[11] generate_tex_elements(str::LaTeXStrings.LaTeXString)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/engine/layout.jl:295
[12] top-level scope
@ REPL[6]:1

caused by: TeXParseError: unsupported command \boldsymbol
at position 12 (string index 12)
\boldsymbol{\nabla}
^
Stack before
[1] TeXExpr :expr

Stacktrace:
[1] _end_command_builder!(stack::DataStructures.Stack{Any}, p::Int64, data::String)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/parser/parser.jl:251
[2] macro expansion
@ ~/.julia/packages/MathTeXEngine/na9MW/src/parser/parser.jl:274 [inlined]
[3] macro expansion
@ ~/.julia/packages/Automa/5enCH/src/codegen.jl:150 [inlined]
[4] texparse(data::String; showdebug::Bool)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/parser/parser.jl:301
[5] texparse
@ ~/.julia/packages/MathTeXEngine/na9MW/src/parser/parser.jl:287 [inlined]
[6] _broadcast_getindex_evalf
@ ./broadcast.jl:683 [inlined]
[7] _broadcast_getindex
@ ./broadcast.jl:656 [inlined]
[8] _getindex
@ ./broadcast.jl:679 [inlined]
[9] _broadcast_getindex
@ ./broadcast.jl:655 [inlined]
[10] getindex
@ ./broadcast.jl:610 [inlined]
[11] copy
@ ./broadcast.jl:912 [inlined]
[12] materialize
@ ./broadcast.jl:873 [inlined]
[13] generate_tex_elements(str::LaTeXStrings.LaTeXString, font_family::MathTeXEngine.FontFamily)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/engine/layout.jl:301
[14] generate_tex_elements(str::LaTeXStrings.LaTeXString)
@ MathTeXEngine ~/.julia/packages/MathTeXEngine/na9MW/src/engine/layout.jl:295
[15] top-level scope
@ REPL[6]:1
```

I'm happy to create a PR, but I don't know where to start.