Moves BootstrapSize type to shared module

[davelopez]

Follow-up to #21635 (comment)

Unifies the definition of the BootstrapSize type by relocating it to a central module for easier reuse and improved type consistency across components.

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[itisAliRH]

We cannot replace the size prop in the Heading because Vue 2's prop validation system expects constructors (like String, Number, or validator functions) rather than TypeScript type aliases. We will replace it later then.

[itisAliRH]

The failed test is irrelevant.

[itisAliRH]

Thank you @davelopez!