Add private field modifier to classes

Add syntax highlighting to the private field modifier `#` as defined in tc39/proposal-class-fields, tc39/proposal-static-class-features and tc39/proposal-private-methods

Author: vieira

syntax/javascript.vim

@@ -141,8 +141,8 @@ syntax region  jsParenRepeat        contained matchgroup=jsParensRepeat        s
 syntax region  jsParenSwitch        contained matchgroup=jsParensSwitch        start=/(/  end=/)/  contains=@jsAll skipwhite skipempty nextgroup=jsSwitchBlock extend fold
 syntax region  jsParenCatch         contained matchgroup=jsParensCatch         start=/(/  end=/)/  skipwhite skipempty nextgroup=jsTryCatchBlock extend fold
 syntax region  jsFuncArgs           contained matchgroup=jsFuncParens          start=/(/  end=/)/  contains=jsFuncArgCommas,jsComment,jsFuncArgExpression,jsDestructuringBlock,jsDestructuringArray,jsRestExpression,jsFlowArgumentDef skipwhite skipempty nextgroup=jsCommentFunction,jsFuncBlock,jsFlowReturn extend fold
-syntax region  jsClassBlock         contained matchgroup=jsClassBraces         start=/{/  end=/}/  contains=jsClassFuncName,jsClassMethodType,jsArrowFunction,jsArrowFuncArgs,jsComment,jsGenerator,jsDecorator,jsClassProperty,jsClassPropertyComputed,jsClassStringKey,jsAsyncKeyword,jsNoise extend fold
-syntax region  jsFuncBlock          contained matchgroup=jsFuncBraces          start=/{/  end=/}/  contains=@jsAll,jsBlock extend fold
+syntax region  jsClassBlock         contained matchgroup=jsClassBraces         start=/{/  end=/}/  contains=jsClassFuncName,jsClassMethodType,jsArrowFunction,jsArrowFuncArgs,jsComment,jsGenerator,jsDecorator,jsClassProperty,jsClassPropertyComputed,jsClassStringKey,jsAsyncKeyword,jsModifier,jsNoise extend fold
+syntax region  jsFuncBlock          contained matchgroup=jsFuncBraces          start=/{/  end=/}/  contains=@jsAll,jsBlock,jsModifier extend fold
 syntax region  jsIfElseBlock        contained matchgroup=jsIfElseBraces        start=/{/  end=/}/  contains=@jsAll,jsBlock extend fold
 syntax region  jsTryCatchBlock      contained matchgroup=jsTryCatchBraces      start=/{/  end=/}/  contains=@jsAll,jsBlock skipwhite skipempty nextgroup=jsCatch,jsFinally extend fold
 syntax region  jsFinallyBlock       contained matchgroup=jsFinallyBraces       start=/{/  end=/}/  contains=@jsAll,jsBlock extend fold
@@ -180,6 +180,7 @@ exe 'syntax match jsArrowFunction /_\ze\s*=>/    skipwhite skipempty nextgroup=j
 " Classes
 syntax keyword jsClassKeyword           contained class
 syntax keyword jsExtendsKeyword         contained extends skipwhite skipempty nextgroup=@jsExpression
+syntax match   jsModifier               contained /\#/
 syntax match   jsClassNoise             contained /\./
 syntax match   jsClassFuncName          contained /\<\K\k*\ze\s*[(<]/ skipwhite skipempty nextgroup=jsFuncArgs,jsFlowClassFunctionGroup
 syntax match   jsClassMethodType        contained /\<\%([gs]et\|static\)\ze\s\+\K\k*/ skipwhite skipempty nextgroup=jsAsyncKeyword,jsClassFuncName,jsClassProperty
@@ -297,6 +298,7 @@ if version >= 508 || !exists("did_javascript_syn_inits")
   HiLink jsOperator             Operator
   HiLink jsOf                   Operator
   HiLink jsStorageClass         StorageClass
+  HiLink jsModifier             StorageClass
   HiLink jsClassKeyword         Keyword
   HiLink jsExtendsKeyword       Keyword
   HiLink jsThis                 Special