[ABNF] Add initial output grammar.

This is for the current simplified version of Leo. When the main function is
executed, it returns a value. According to this grammar, the output (i.e.
`.out`) file contains a single literal, which describes the output value,
preceded by an `[output]` title. This may be extended in the future.

docs/grammar/README.md

@@ -963,4 +963,45 @@ Go to: _[input-title](#user-content-input-title)_;
 <a name="input-file"></a>
 ```abnf
 input-file = *input-section
+```
+
+
+--------
+
+
+Output Grammar
+--------------
+
+<a name="output-expression"></a>
+```abnf
+output-expression = literal
+```
+
+Go to: _[literal](#user-content-literal)_;
+
+
+<a name="output-item"></a>
+```abnf
+output-item = output-expression ";"
+```
+
+Go to: _[output-expression](#user-content-output-expression)_;
+
+
+<a name="output-title"></a>
+```abnf
+output-title = "[" %s"output" "]"
+```
+
+<a name="output-section"></a>
+```abnf
+output-section = output-title output-item
+```
+
+Go to: _[output-item](#user-content-output-item), [output-title](#user-content-output-title)_;
+
+
+<a name="output-file"></a>
+```abnf
+output-file = output-section
 

docs/grammar/abnf-grammar.txt

@@ -374,3 +374,18 @@ input-title = "[" ( %s"public" / %s"private" / %s"constant" / %s"const" ) "]"
 input-section = input-title *input-item
 
 input-file = *input-section
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+; Output Grammar
+; --------------
+
+output-expression = literal
+
+output-item = output-expression ";"
+
+output-title = "[" %s"output" "]"
+
+output-section = output-title output-item
+
+output-file = output-section