add two-fer exercise

Author: TheRealOwenRees

exercises/practice/two-fer/.docs/instructions.md

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+# Instructions
+
+Your task is to determine what you will say as you give away the extra cookie.
+
+If you know the person's name (e.g. if they're named Do-yun), then you will say:
+
+```text
+One for Do-yun, one for me.
+```
+
+If you don't know the person's name, you will say _you_ instead.
+
+```text
+One for you, one for me.
+```
+
+Here are some examples:
+
+| Name   | Dialogue                    |
+| :----- | :-------------------------- |
+| Alice  | One for Alice, one for me.  |
+| Bohdan | One for Bohdan, one for me. |
+|        | One for you, one for me.    |
+| Zaphod | One for Zaphod, one for me. |

exercises/practice/two-fer/.docs/introduction.md

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+# Introduction
+
+In some English accents, when you say "two for" quickly, it sounds like "two fer".
+Two-for-one is a way of saying that if you buy one, you also get one for free.
+So the phrase "two-fer" often implies a two-for-one offer.
+
+Imagine a bakery that has a holiday offer where you can buy two cookies for the price of one ("two-fer one!").
+You take the offer and (very generously) decide to give the extra cookie to someone else in the queue.

exercises/practice/two-fer/.meta/TwoFer.res

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+let twoFer = name => {
+  switch name {
+  | None => "One for you, one for me."
+  | Some(nameString) => "One for " ++ nameString ++ ", one for me."
+  }
+}

exercises/practice/two-fer/.meta/config.json

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+{
+  "authors": [
+    "tejasbubane",
+    "therealowenrees"
+  ],
+  "files": {
+    "solution": [
+      "src/TwoFer.res"
+    ],
+    "test": [
+      "tests/TwoFer_test.res"
+    ],
+    "example": [
+      ".meta/TwoFer.res"
+    ],
+    "editor": [
+      "src/TwoFer.resi"
+    ]
+  },
+  "blurb": "Create a sentence of the form \"One for X, one for me.\".",
+  "source_url": "https://github.com/exercism/problem-specifications/issues/757"
+}

exercises/practice/two-fer/.meta/tests.toml

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+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[1cf3e15a-a3d7-4a87-aeb3-ba1b43bc8dce]
+description = "no name given"
+
+[b4c6dbb8-b4fb-42c2-bafd-10785abe7709]
+description = "a name given"
+
+[3549048d-1a6e-4653-9a79-b0bda163e8d5]
+description = "another name given"

exercises/practice/two-fer/rescript.json

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+{
+  "name": "@exercism/rescript",
+  "sources": [
+    { "dir": "src", "subdirs": true, "type": "dev" },
+    { "dir": "tests", "subdirs": true, "type": "dev" }
+  ],
+  "package-specs": [
+    {
+      "module": "esmodule",
+      "in-source": true
+    }
+  ],
+  "suffix": ".res.js",
+  "dev-dependencies": ["rescript-test"]
+}

exercises/practice/two-fer/src/TwoFer.res

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+let twoFer: option<string> => string

exercises/practice/two-fer/src/TwoFer.resi

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+open Test
+open TwoFer
+
+let stringEqual = (~message=?, a: string, b: string) => assertion(~message?, ~operator="stringEqual", (a, b) => a == b, a, b)
+
+test("no name given", () => {
+  stringEqual(~message="no name given", twoFer(None), "One for you, one for me.")
+})
+
+test("a name given", () => {
+  stringEqual(~message="a name given", twoFer(Some("Alice")), "One for Alice, one for me.")
+})
+
+test("another name given", () => {
+  stringEqual(~message="another name given", twoFer(Some("Bob")), "One for Bob, one for me.")
+})
+