Implement read --delimiter

This takes a string that is then split upon like `string split`.

Unlike $IFS, the string is used as one piece, not a set of characters.

There is still a fallback to IFS if no delimiter is given, that
behaves exactly as before.

Fixes #4156.

doc_src/read.txt

@@ -13,6 +13,8 @@ The following options are available:
 
 - `-c CMD` or `--command=CMD` sets the initial string in the interactive mode command buffer to `CMD`.
 
+- `-d DELIMITER` or `--delimiter=DELIMITER` splits on DELIMITER.
+
 - `-g` or `--global` makes the variables global.
 
 - `-i` or `--silent` makes the characters typed obfuscated. This is useful for reading things like passwords or other sensitive information. Note that in bash the short flag is `-s`. We can't use that due to the existing use as an alias for `--shell`.
@@ -39,7 +41,7 @@ The following options are available:
 
 - `-z` or `--null` reads up to NUL instead of newline. Disables interactive mode.
 
-`read` reads a single line of input from stdin, breaks it into tokens based on the `IFS` shell variable, and then assigns one token to each variable specified in `VARIABLES`. If there are more tokens than variables, the complete remainder is assigned to the last variable. As a special case, if `IFS` is set to the empty string, each character of the input is considered a separate token.
+`read` reads a single line of input from stdin, breaks it into tokens based on the delimiter set via `-d`/`--delimiter` as a complete string (like `string split` or, if that has not been given the (deprecated) `IFS` shell variable as a set of characters, and then assigns one token to each variable specified in `VARIABLES`. If there are more tokens than variables, the complete remainder is assigned to the last variable. As a special case, if `IFS` is set to the empty string, each character of the input is considered a separate token.
 
 If `-a` or `--array` is provided, only one variable name is allowed and the tokens are stored as an array in this variable.
 
@@ -64,4 +66,11 @@ echo hello|read foo
 printf '%s\n' line1 line2 line3 line4 | while read -l foo
                   echo "This is another line: $foo"
               end
+
+# Delimiters given via "-d" are taken as one string
+echo a==b==c | read -d == -l a b c
+echo $a # a
+echo $b # b
+echo $c # c
+
 \endfish

doc_src/string.txt

@@ -91,6 +91,8 @@ Exit status: 0 if at least one replacement was performed, or 1 otherwise.
 
 `string split` splits each STRING on the separator SEP, which can be an empty string. If `-m` or `--max` is specified, at most MAX splits are done on each STRING. If `-r` or `--right` is given, splitting is performed right-to-left. This is useful in combination with `-m` or `--max`. Exit status: 0 if at least one split was performed, or 1 otherwise.
 
+See also `read --delimiter`.
+
 \subsection string-sub "sub" subcommand
 
 `string sub` prints a substring of each string argument. The start of the substring can be specified with `-s` or `--start` followed by a 1-based index value. Positive index values are relative to the start of the string and negative index values are relative to the end of the string. The default start value is 1. The length of the substring can be specified with `-l` or `--length`. If the length is not specified, the substring continues to the end of each STRING. Exit status: 0 if at least one substring operation was performed, 1 otherwise.