Convert src/core/charsets.js and src/core/standard_fonts.js to ES6 syntax

Moreover, include the "no var" ESLint comment to
`src/core/annotation.js` and `src/core/ps_parser.js` since they are
already converted.

src/core/charsets.js

@@ -12,8 +12,9 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint no-var: error */
 
-var ISOAdobeCharset = [
+const ISOAdobeCharset = [
   '.notdef', 'space', 'exclam', 'quotedbl', 'numbersign', 'dollar',
   'percent', 'ampersand', 'quoteright', 'parenleft', 'parenright',
   'asterisk', 'plus', 'comma', 'hyphen', 'period', 'slash', 'zero',
@@ -50,7 +51,7 @@ var ISOAdobeCharset = [
   'ugrave', 'yacute', 'ydieresis', 'zcaron'
 ];
 
-var ExpertCharset = [
+const ExpertCharset = [
   '.notdef', 'space', 'exclamsmall', 'Hungarumlautsmall', 'dollaroldstyle',
   'dollarsuperior', 'ampersandsmall', 'Acutesmall', 'parenleftsuperior',
   'parenrightsuperior', 'twodotenleader', 'onedotenleader', 'comma',
@@ -90,7 +91,7 @@ var ExpertCharset = [
   'Ydieresissmall'
 ];
 
-var ExpertSubsetCharset = [
+const ExpertSubsetCharset = [
   '.notdef', 'space', 'dollaroldstyle', 'dollarsuperior',
   'parenleftsuperior', 'parenrightsuperior', 'twodotenleader',
   'onedotenleader', 'comma', 'hyphen', 'period', 'fraction',