Fix bug in extracting variable values containing | (#81)

The bug happens whenever the variable being previewed contains a |. What happens is that the third regex used in __fzf_extract_var_info is greedy and will over-remove text up to the second to last |. For example:
$ set variable "a | b | c | d"
$ __fzf_extract_var_info variable (set --show | psub)
set in global scope, unexported, with 1 elements
[1]  d

By making the third regex lazy, it now works properly:
$ set variable "a | b | c | d"
$ __fzf_extract_var_info variable (set --show | psub)
set in global scope, unexported, with 1 elements
[1] a | b | c | d

Additionally, I've added a Fishtape test to make debugging these tricky regexes more easily in the future, which I expect to need to do as the master branch of Fish has already changed the output of set --show.

functions/__fzf_extract_var_info.fish

@@ -6,7 +6,7 @@ function __fzf_extract_var_info --argument-names variable_name set_show_output -
     # $variable_name[
     string match --entire --regex "^\\\$$variable_name(?:: set|\[)" <$set_show_output |
 
-    # Strip the variable name from the variable info, replacing...
+    # Strip the variable name from the scope info, replacing...
     # $variable_name: set in global scope
     # ...with...
     # set in global scope
@@ -16,7 +16,7 @@ function __fzf_extract_var_info --argument-names variable_name set_show_output -
     # $variable_name[1]: length=14 value=|variable_value|
     # ...with...
     # [1] variable_value
-    string replace --regex "^\\\$$variable_name(\[.+\]).+\|(.+)\|\$" '\$1 \$2'
+    string replace --regex "^\\\$$variable_name(\[\d+\]).+?\|(.+)\|\$" '\$1 \$2'
 
     # Final output example for $PATH:
     # set in global scope, unexported, with 5 elements

tests/extract_var_info_test.fish

@@ -0,0 +1,7 @@
+@echo === extract_var_info ===
+
+set --local variable "| a | b | c | d |"
+set --local actual (__fzf_extract_var_info variable (set --show | psub) | string collect)
+set --local expected "set in local scope, unexported, with 1 elements
+[1] | a | b | c | d |"
+@test "vars containing '|'" $actual = $expected