Create script to automatically bump the version in .version (#147)

Initialize the `script/` directory dedicated to scripts used during
development. The only script currently present automates the process of
updating the `.version` file for a release based on the argument that is
provided ('major', 'minor', or 'patch'). The Release Guidelines have
been updated to prefer the use of this script when creating a release,
maintaining the manual instructions in case the script doesn't work.

The script is intentionally given the MIT-0 (aka 'no attribution')
license identifier as the intent is for it to be easier to re-use, that
is without requiring attribution upon re-use. The intend is that all
homegrown scripts will use this less restrictive license.

.gitignore

@@ -57,6 +57,11 @@
 /lib/**
 !/lib/*.sh
 
+## Script
+!/script/
+/script/**
+!/script/*.sh
+
 ## Test
 !/spec/
 /spec/**

.shellspec

@@ -8,6 +8,6 @@
 
 ## kcov (coverage)
 --covdir _coverage/
---kcov-options "--exclude-pattern=/_coverage/,/_report/,/spec/,/.shellcheckrc,/.shellspec"
+--kcov-options "--exclude-pattern=/_coverage/,/_report/,/script/,/spec/,/.shellcheckrc,/.shellspec"
 --kcov-options "--include-path=. --path-strip-level=1"
 --kcov-options "--include-pattern=.sh"

RELEASE.md

@@ -18,8 +18,13 @@ found in this file (using v0.1.2 as an example):
    git clone git@github.com:ericcornelissen/tool-versions-update-action.git
    ```
 
-1. Update the version number in the `.version` file following [Semantic
-   Versioning]:
+1. Update the version number following [Semantic Versioning]:
+
+   ```shell
+   ./script/version-bump.sh [major|minor|patch]
+   ```
+
+   Or edit the `.version` file manually:
 
    ```diff
    - 0.1.1

script/version-bump.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+# SPDX-License-Identifier: MIT-0
+
+# --- Setup ------------------------------------------------------------------ #
+
+set -eo pipefail
+
+type="$1"
+
+version=$(grep -o '[^-]*$' <.version)
+major=$(echo "${version}" | cut -d. -f1)
+minor=$(echo "${version}" | cut -d. -f2)
+patch=$(echo "${version}" | cut -d. -f3)
+
+# --- Script ----------------------------------------------------------------- #
+
+if [[ "${type}" == "patch" ]]; then
+	patch=$((patch + 1))
+elif [[ "${type}" == "minor" ]]; then
+	minor=$((minor + 1))
+	patch=0
+elif [[ "${type}" == "major" ]]; then
+	major=$((major + 1))
+	minor=0
+	patch=0
+elif [[ -z "${type}" ]]; then
+	echo "Provide an update type, one of 'major', 'minor', or 'patch'"
+	exit 1
+else
+	echo "Unknown update type '${type}'"
+	exit 1
+fi
+
+echo "${major}.${minor}.${patch}" >.version