Add dependabot.yml for dependency version updates

[kanwalpreetd]

What

Adding base config for dependabot.yml, which enables automatic PRs for dependency version updates through Dependabot. This base config sets version updates to run on a weekly schedule, groups PRs for version updates according to major vs minor/patch updates, and sets max PRs that Dependabot can create for version updates per package-ecosystem.
But if you would like, several config options can be added to dependabot.yml as per configuration-options-for-the-dependabot.yml-file to customize version updates, such as:

• Group PRs for version updates by different criteria
• Set dependency-type option in allow option to all for enabling version updates for both direct and indirect dependencies (as well as prod and dev dependencies). By default, PRs for only updating direct prod+dev dependencies versions are created.
• Set reviewers for Dependabot version updates PRs
• Etc.

Also, automatic PRs for security updates through Dependabot are enabled through a global setting. PRs for security updates will be grouped as much as possible across directories and per ecosystem through the global setting (grouping-dependabot-security-updates). But dependabot.yml can also be used for more granular security updates settings in addition to version updates (some of the options in dependabot.yml are shared across security and version updates), such as:

• Grouping security updates according to custom criteria: overriding-the-default-behavior-with-a-configuration-file
• Only allowing automatic PRs for security updates for certain dependencies by configuring allow directive to a certain value such as production, development, direct, indirect. By default, security updates are applied to all dependencies (direct, indirect, production, development).
• Etc.

Why

To enable using the latest package versions, and provide a way to customize Dependabot security updates PRs.

[Shaptic]

Can I ask how this is different from GitHub's built-in dependabot support for this repo? See here: https://github.com/stellar/js-stellar-sdk/security/dependabot. It regularly opens PRs for upgrading problematic dependencies, e.g. #906.

[kanwalpreetd]

@Shaptic This enables Dependabot to create auto PRs for upgrading dependency versions, if newer versions are available.

Through the currently enabled Dependabot setting, Dependabot only creates PRs, such as 906, for dependencies which have an associated security alert (83).

So, this will enable auto PRs for updating all outdated dependency versions, not just the ones which have a security alert against them.

[Shaptic]

@kanwalpreetd thanks for the explanation! I probably should've read the PR description more closely 😅