Yacc/Bison: Fix the $ symbol and update syntax for Bison

Summary:
Hi
The Yacc/Bison file highlights as 'dsError' the '$' symbol (it is used as pseudo-variable). The problem comes from the 'isocpp.xml' file of C++, which highlights it as 'Error' (probably, this was added to the C++ file later).
**data/syntax/yacc.xml** (lines 131 & 132):
   131|      <IncludeRules context="##C++" />
   132|      <DetectChar attribute="Directive" context="Dol" char="$" />
   133|  </context>
**autotests/reference/highlight.y.ref** :
   35|  […] <Symbol>(</Symbol><Error>$</Error><Deimal>2</Decimal><Symbol>) [...]
   40|  <Normal Text> </Normal Text><Error>$$</Error><Normal Text> </Normal Text> […]

That is corrected. Also, this file is outdated for Bison. Therefore, it is updated according to the parser of the latest version of Bison (3.0.4).

**Changes/fixes for Bison:**
* Add declarations that contain blocks of code that are not currently highlighted. For example:
```
%code { C++ code... }
%destructor { C++ code, with symbols $… }
%initial-action { C++ code, with symbols $… }
```

* Add the Bison variable '@' (@$, @n, @[name], $@n).
* Add directives in rules: %?{ expr…},  %empty, %prec, %dprec and %merge.
* Allow a tag in %union declarations. Ex:
```
%union tag { code … }
```

* Allow grammar declarations in the grammar rules section, such as: %union, %code, %type, etc.
* Allow declarations in multiple lines. Ex:
```
%token
         PERCENT_DEBUG           "%debug"
         PERCENT_DEFAULT_PREC    "%default-prec"
;
```
* The ';' char is not necessary to finish a rule. Allow ';' followed by ‘|’, within a rule (See line 603 in the “src/parse-gram.y” file, from the Bison 3.0.4 source [2]).
* Add the mimetypes (text/x-yacc, text/x-bison) and the extensions '.ypp' & '.y++' [3].

I hope these changes are useful.
Any inconvenience (as if it is necessary to contact the original author of the file to approve the changes), do not hesitate to notify.

Source:
[1] https://www.gnu.org/software/bison/manual/html_node/Table-of-Symbols.html
[2] https://ftp.gnu.org/gnu/bison/bison-3.0.4.tar.xz ( ./src/parse-gram.y )
[3] https://www.gnu.org/software/bison/manual/html_node/Invocation.html

Test Plan: I've been testing these changes the last week and I think it's ready.

Reviewers: #framework_syntax_highlighting, dhaumann

Reviewed By: #framework_syntax_highlighting, dhaumann

Subscribers: dhaumann, #framework_syntax_highlighting, #frameworks

Tags: #frameworks, #framework_syntax_highlighting

Differential Revision: https://phabricator.kde.org/D10783