Add findstr completion (#8481)

* Add findstr completion

* Standardize completion

* Show completion only on Windows

* Use single quotes where possible

* Remove quotes where possible

* Remove OS check

* Use single quotes for --arguments

share/completions/findstr.fish

@@ -0,0 +1,45 @@
+function __findstr_generate_args --description 'Function to generate args'
+  set --local current_token (commandline --current-token --cut-at-cursor)
+  switch $current_token
+    case '/a:*'
+      echo -e '0\tBlack
+1\tBlue
+2\tGreen
+3\tAqua
+4\tRed
+5\tPurple
+6\tYellow
+7\tWhite
+8\tGray
+9\tLight blue
+A\tLight green
+B\tLight aqua
+C\tLight red
+D\tLight purple
+E\tLight yellow
+F\tBright white' | awk -F '\t' "{ printf \"$current_token%s\t%s\n\", \$1, \$2 }"
+    case '*'
+      echo -e '/b\tMatch at the beginning of the line
+/e\tMatch at the end of the line
+/l\tUse literal strings
+/r\tUse regular expressions
+/s\tSearch a current directory and all subdirectories
+/i\tIgnore case
+/x\tMatch exactly
+/v\tInvert match
+/n\tPrint line numbers
+/m\tPrint only the file name with matching lines
+/o\tPrint character offset
+/p\tSkip files with non-printable characters
+/off\tDon\'t skip files that have the offline attribute set
+/offline\tDon\'t skip files that have the offline attribute set
+/f:\tRead a file list from file
+/c:\tSpecify a literal search string
+/g:\tRead literal search strings from file
+/d:\tSpecify the search list of directories
+/a:\tUse color attributes with two hexadecimal digits
+/?\tShow help'
+  end
+end
+
+complete --command findstr --no-files --arguments '(__findstr_generate_args)'