lib: remove utf8proc dependency (#436)

* Remove dependency on utf8proc

This removes the only external dependency on utf8proc for UTF-8 decoding. It does so by implementing its own UTF-8 decoder. This decoder is both faster and has a simpler API.

 * .gitmodules: remove utf8proc submodule
 * docs/section-2-using-parsers.md: remove requirement for utf8proc submodule
 * docs/section-6-contributing.md: likewise
 * lib/Cargo.toml: remove utf8proc subdirectory package include
 * lib/README.md: remove utf8proc subdirectory description
 * lib/binding_rust/build.rs: remove utf8proc compiler include directory
 * lib/src/lexer.c: remove utf8proc dependencies and types
 * lib/src/lib.c: remove utf8proc dependency
 * lib/src/unicode.h: define types for Unicode decoders
 * lib/src/utf16.{c,h}: implement more readable UTF-16 decoder
 * lib/src/utf8.{c,h}: implement fast UTF-8 decoder
 * lib/utf8proc: remove utf8proc submodule directory
 * script/build-lib: remove utf8proc compiler include directory
 * script/build-wasm: likewise

* Optimize ts_lexer__get_lookahead.

Try to favor non-failure code path and assign lookahead values directly to lexer

 * lib/src/lexer.c: optimize for non-failure code path

* Fix some compiler errors

 * lib/src/lexer.c: cast from signed to unsigned for decode_next result
 * lib/src/utf16.c: fix non-constant initializers for older compilers

* Remove some missed remnants of utf8proc

 * docs/section-2-using-parsers.md: only two include paths necessary now
 * lib/src/lib.c: no need to define UTF8PROC_STATIC

* Use ICU's utf8 and utf16 decoding routines

* Remove unnecessary casts when calling icu macros

* Check buffer length before attempting to decode a unicode character

* Use new unicode function when parsing Queries

Co-Authored-By: Matthew Krupcale <mkrupcale@matthewkrupcale.com>

* Mark libicu files as vendored for GitHub's stats

script/build-lib

@@ -14,7 +14,6 @@ ${CC}                   \
   $CFLAGS               \
   -I lib/src            \
   -I lib/include        \
-  -I lib/utf8proc       \
   lib/src/lib.c     \
   -o tree-sitter.o
 

script/build-wasm

@@ -94,7 +94,6 @@ $emcc                                \
   -D NDEBUG=                         \
   -I lib/src                         \
   -I lib/include                     \
-  -I lib/utf8proc                    \
   --js-library ${web_dir}/imports.js \
   --pre-js ${web_dir}/prefix.js      \
   --post-js ${web_dir}/binding.js    \