chore: Added .devcontainer for GitHub Codespaces. (#300)

* Added .devcontainer for GitHub Codespaces.

* Changed packages that will be installed.

.devcontainer/Dockerfile

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+FROM mcr.microsoft.com/vscode/devcontainers/base:ubuntu-22.04
+
+ADD first-run-notice.txt /usr/local/etc/vscode-dev-containers/first-run-notice.txt
+
+RUN apt-get update -y && \
+    apt-get install -y php php-curl php-xml inkscape composer

.devcontainer/devcontainer.json

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+{
+  "build": {
+    "dockerfile": "Dockerfile"
+  },
+  "onCreateCommand": "/workspaces/github-readme-streak-stats/.devcontainer/on-create.sh",
+  "postCreateCommand": "/workspaces/github-readme-streak-stats/.devcontainer/post-create.sh"}

.devcontainer/first-run-notice.txt

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+👋 Welcome to Codespaces! You are using the pre-configured image.
+
+Tests can be executed with: composer test

.devcontainer/on-create.sh

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+#!/bin/bash
+set -e
+
+cd /workspaces/github-readme-streak-stats
+composer install

.devcontainer/post-create.sh

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+#!/bin/bash
+set -e
+
+cd /workspaces/github-readme-streak-stats
+if [ -n "$GITHUB_TOKEN" ]; then
+  echo "TOKEN=$GITHUB_TOKEN" > .env
+fi