cli: add engine flag

This permits switching between the different regex engine modes that
ripgrep supports. The purpose of this flag is to make it easier to
extend ripgrep with additional regex engines.

Closes #1488, Closes #1502

complete/_rg

@@ -78,6 +78,13 @@ _rg() {
     {-E+,--encoding=}'[specify text encoding of files to search]: :_rg_encodings'
     $no'--no-encoding[use default text encoding]'
 
+    + '(engine)' # Engine choice options
+    '--engine=[select which regex engine to use]:when:((
+      default\:"use default engine"
+      pcre2\:"identical to --pcre2"
+      auto\:"identical to --auto-hybrid-regex"
+    ))'
+
     + file # File-input options
     '(1)*'{-f+,--file=}'[specify file containing patterns to search for]: :_files'