patch 9.0.1668: PEM files are not recognized

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

Author: ObserverOfTime

runtime/filetype.vim

@@ -1511,6 +1511,9 @@ au BufNewFile,BufRead *.pdf				setf pdf
 " PCMK - HAE - crm configure edit
 au BufNewFile,BufRead *.pcmk				setf pcmk
 
+" PEM (Privacy-Enhanced Mail)
+au BufNewFile,BufRead *.pem,*.cer,*.crt,*.csr		setf pem
+
 " Perl
 if has("fname_case")
   au BufNewFile,BufRead *.pl,*.PL			call dist#ft#FTpl()

src/testdir/test_filetype.vim

@@ -506,6 +506,7 @@ def s:GetFilenameChecks(): dict<list<string>>
     pccts: ['file.g'],
     pcmk: ['file.pcmk'],
     pdf: ['file.pdf'],
+    pem: ['file.pem', 'file.cer', 'file.crt', 'file.csr'],
     perl: ['file.plx', 'file.al', 'file.psgi', 'gitolite.rc', '.gitolite.rc', 'example.gitolite.rc', '.latexmkrc', 'latexmkrc'],
     pf: ['pf.conf'],
     pfmain: ['main.cf', 'main.cf.proto'],

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1668,
 /**/
     1667,
 /**/