Allow a non-ASCII character to start a name that is not a type name

The rules above this one already take a non-ASCII character anywhere but the
first position, and the ones for symbols, instance variables and class
variables take it there too. What is left is a bare identifier, and there the
first character is not just a character: RBS reads its case to tell a class
name from an interface name from an alias name.

`rbs_next_char` reports every character outside ASCII as one and the same code
point, so a rule cannot read that case on its own. The encoding could be asked,
and its answer would be right -- the tables in `rbs_encoding.c` are a copy of
prism's, and prism is what decides this in Ruby, so the two agree on every
character in every encoding rbs knows.

`TypeName#kind` is the problem. It reads `[[:upper:]]`, which is Onigmo's
table, not prism's, and the two are different functions: they disagree about
891 characters, among them the full-width capitals of every EUC-JP and
Shift_JIS variant, which Ruby does start a constant on. Agreeing would mean
carrying prism's table on the Ruby side as well -- 662 code point ranges, 75
single-byte tables and the Japanese byte ranges, kept in step with the C copy
forever.

So RBS takes less than Ruby gives. A name whose kind is read from its first
character has to open with ASCII, and every other name may open with whatever
Ruby accepts. Those names get a token of their own: the parser takes
`tNONASCIIIDENT` for a method name, a parameter name and a keyword, and
nowhere a type name is read, so `class Foo日本語` parses and `class 日本語`
does not.

An incorrect `byte_range` lands on such a name, so the token it is reported
against is a real one now instead of `ErrorToken`.

Author: soutaro

include/rbs/lexer.h

@@ -72,18 +72,19 @@ enum RBSTokenType {
     kRETURN,       /* return */
     kMODULESELF,   /* module-self */
 
-    tLIDENT,    /* Identifiers starting with lower case */
-    tUIDENT,    /* Identifiers starting with upper case */
-    tULIDENT,   /* Identifiers starting with `_` followed by upper case */
-    tULLIDENT,  /* Identifiers starting with `_` followed by lower case */
-    tGIDENT,    /* Identifiers starting with `$` */
-    tAIDENT,    /* Identifiers starting with `@` */
-    tA2IDENT,   /* Identifiers starting with `@@` */
-    tBANGIDENT, /* Identifiers ending with `!` */
-    tEQIDENT,   /* Identifiers ending with `=` */
-    tQIDENT,    /* Quoted identifier */
-    pAREF_OPR,  /* [] */
-    tOPERATOR,  /* Operator identifier */
+    tLIDENT,        /* Identifiers starting with lower case */
+    tUIDENT,        /* Identifiers starting with upper case */
+    tULIDENT,       /* Identifiers starting with `_` followed by upper case */
+    tULLIDENT,      /* Identifiers starting with `_` followed by lower case */
+    tNONASCIIIDENT, /* Identifiers starting with a character outside ASCII */
+    tGIDENT,        /* Identifiers starting with `$` */
+    tAIDENT,        /* Identifiers starting with `@` */
+    tA2IDENT,       /* Identifiers starting with `@@` */
+    tBANGIDENT,     /* Identifiers ending with `!` */
+    tEQIDENT,       /* Identifiers ending with `=` */
+    tQIDENT,        /* Quoted identifier */
+    pAREF_OPR,      /* [] */
+    tOPERATOR,      /* Operator identifier */
 
     tCOMMENT,       /* Comment */
     tLINECOMMENT,   /* Comment of all line */