eliminate dependency on Base.is_id_char and julia-specific utf8proc

As discussed in #372, in order to make the JuliaSyntax parsing independent of the version of Julia in which it runs, it needs to:

1. Add a dependency on [utf8proc_jll](https://github.com/JuliaBinaryWrappers/utf8proc_jll.jl), rather than using the utf8proc bundled with Julia. This is important because the supported version of Unicode (for character classification) depends on utf8proc.
2. Re-implement `Base.is_id_start_char` and `Base.is_id_char` to identify which characters can appear in identifiers (or technically, which characters can begin graphemes, now that #372 is merged … this could simplify the rules slightly since we no longer need to explicitly check for combining marks).

Start by reviewing #372 and the JuliaSyntax parsing code that uses Julia's bundled utf8proc and Base.is_id_start_char/Base.is_id_char. Done means adding the utf8proc_jll dependency and replacing those Base checks with identifier or grapheme-start classification independent of the Julia version.

Written by the indexing model from the issue text.