Do not override previous filetype for *.tsx

There are other plugins (like ianks/vim-tsx) that set their own filetype for
*.tsx files and we don't want to override them. Using `setfiletype` does that
by only setting the filetype if it has not already been set.

For *.ts files we continue to use `set filetype` as that will overwrite the
default for *.ts files.

fix leafgarland#110
close leafgarland#111

Author: leafgarland

ftdetect/typescript.vim

@@ -1 +1,4 @@
-autocmd BufNewFile,BufRead *.ts setlocal filetype=typescript
+" use `set filetype` to override default filetype=xml for *.ts files
+autocmd BufNewFile,BufRead *.ts  set filetype=typescript
+" use `setfiletype` to not override any other plugins like ianks/vim-tsx
+autocmd BufNewFile,BufRead *.tsx setfiletype typescript