vim-patch:9.0.1668: PEM files are not recognized (#24169)

Problem:    PEM files are not recognized.
Solution:   Add patterns to match PEM files. (closes vim/vim#12582)

0256d76a33

runtime/lua/vim/filetype.lua

@@ -783,6 +783,10 @@ local extension = {
   g = 'pccts',
   pcmk = 'pcmk',
   pdf = 'pdf',
+  pem = 'pem',
+  cer = 'pem',
+  crt = 'pem',
+  csr = 'pem',
   plx = 'perl',
   prisma = 'prisma',
   psgi = 'perl',