copy subset of Sprig template functions

docs/sprig/integer_slice.md

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+# Integer List Functions
+
+## until
+
+The `until` function builds a range of integers.
+
+```
+until 5
+```
+
+The above generates the list `[0, 1, 2, 3, 4]`.
+
+This is useful for looping with `range $i, $e := until 5`.
+
+## untilStep
+
+Like `until`, `untilStep` generates a list of counting integers. But it allows
+you to define a start, stop, and step:
+
+```
+untilStep 3 6 2
+```
+
+The above will produce `[3 5]` by starting with 3, and adding 2 until it is equal
+or greater than 6. This is similar to Python's `range` function.
+
+## seq
+
+Works like the bash `seq` command.
+* 1 parameter  (end) - will generate all counting integers between 1 and `end` inclusive.
+* 2 parameters (start, end) - will generate all counting integers between `start` and `end` inclusive incrementing or decrementing by 1.
+* 3 parameters (start, step, end) - will generate all counting integers between `start` and `end` inclusive incrementing or decrementing by `step`.
+
+```
+seq 5       => 1 2 3 4 5
+seq -3      => 1 0 -1 -2 -3
+seq 0 2     => 0 1 2
+seq 2 -2    => 2 1 0 -1 -2
+seq 0 2 10  => 0 2 4 6 8 10
+seq 0 -2 -5 => 0 -2 -4
+```