docs: add Japanese translations for all chapters

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+# 第12章の文法
+
+[English](12-grammar.md) | 日本語
+
+この文書は [原文](12-grammar.md) の日本語訳です。
+
+[第12章に戻る](../README.ja.md)
+
+```antlrv4
+grammar SimpleLanguage;
+
+program
+    : statement+ EOF
+    ;
+
+statement
+    : returnStatement
+    | structDeclaration
+    | declStatement
+    | blockStatement
+    | expressionStatement
+    | ifStatement
+    | whileStatement
+    | breakStatement
+    | continueStatement
+    ;
+
+PRIMTYPE
+    : 'int'
+    ;
+
+field
+    : PRIMTYPE IDENTIFIER ';'
+    | structName ('?')? IDENTIFIER ';'
+    ;
+
+fields
+    : field+
+    ;
+
+structDeclaration
+    : 'struct' IDENTIFIER '{' fields '}'
+    ;
+
+whileStatement
+    : 'while' '(' expression ')' statement
+    ;
+
+breakStatement
+    : 'break' ';'
+    ;
+
+continueStatement
+    : 'continue' ';'
+    ;
+
+ifStatement
+    : 'if' '(' expression ')' statement ('else' statement)?
+    ;
+
+
+expressionStatement
+    : IDENTIFIER '=' expression ';'
+    | fieldExpression '=' expression ';'
+    ;
+
+blockStatement
+    : '{' statement+ '}'
+    ;
+
+structName
+    : IDENTIFIER
+    ;
+
+declStatement
+    : PRIMTYPE IDENTIFIER '=' expression ';'
+    | structName ('?')? IDENTIFIER '=' expression ';'
+    ;
+
+returnStatement
+    : 'return' expression ';'
+    ;
+
+expression
+    : comparisonExpression
+    ;
+
+comparisonExpression
+    : additiveExpression (('==' | '!='| '>'| '<'| '>='| '<=') additiveExpression)*
+    ;
+
+additiveExpression
+    : multiplicativeExpression (('+' | '-') multiplicativeExpression)*
+    ;
+
+multiplicativeExpression
+    : unaryExpression (('*' | '/') unaryExpression)*
+    ;
+
+unaryExpression
+    : ('-') unaryExpression
+    | '!' unaryExpression
+    | primaryExpression
+    ;
+
+newExpression
+    : 'new' IDENTIFIER
+    ;
+
+fieldExpression
+    : primaryExpression '.' IDENTIFIER
+    ;
+
+primaryExpression
+    : IDENTIFIER
+    | INTEGER_LITERAL
+    | 'true'
+    | 'false'
+    | 'null'
+    | newExpression
+    | '(' expression ')'
+    | fieldExpression
+    ;
+
+INTEGER_LITERAL
+    : [1-9][0-9]*
+    | [0]
+    ;
+
+IDENTIFIER
+    : NON_DIGIT (NON_DIGIT | DEC_DIGIT)*
+    ;
+
+NON_DIGIT: [a-zA-Z_];
+DEC_DIGIT: [0-9];
+```