fix: add missing TSSymbolKeyword type

[ikatyang]

Context: prettier/prettier#2851

[jsf-clabot]

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[azz]

I added never in #381, I wonder if there's more.

[j-f1]

@azz Of the *Keyword nodes, these ones are missing:

There’s lots 🙁

TSAsKeyword
TSAwaitKeyword
TSBreakKeyword
TSCaseKeyword
TSCatchKeyword
TSClassKeyword
TSConstKeyword
TSConstructorKeyword
TSContinueKeyword
TSDebuggerKeyword
TSDefaultKeyword
TSDeleteKeyword
TSDoKeyword
TSElseKeyword
TSEnumKeyword
TSExtendsKeyword
TSFalseKeyword
TSFinallyKeyword
TSFirstKeyword
TSForKeyword
TSFromKeyword
TSFunctionKeyword
TSGetKeyword
TSGlobalKeyword
TSIfKeyword
TSImplementsKeyword
TSImportKeyword
TSInKeyword
TSInstanceOfKeyword
TSInterfaceKeyword
TSIsKeyword
TSKeyOfKeyword
TSLastKeyword
TSLetKeyword
TSModuleKeyword
TSNamespaceKeyword
TSNewKeyword
TSOfKeyword
TSPackageKeyword
TSRequireKeyword
TSReturnKeyword
TSSetKeyword
TSSuperKeyword
TSSwitchKeyword
TSThisKeyword
TSThrowKeyword
TSTrueKeyword
TSTryKeyword
TSTypeKeyword
TSTypeOfKeyword
TSVarKeyword
TSWhileKeyword
TSWithKeyword
TSYieldKeyword

However, I’m not sure if they all actually appear in the AST.

[JamesHenry]

Thanks a lot for contributing, @ikatyang!