language: Add configuration option to disable runnables

[AlvaroParker]

Objective

• Fixes Test run gutter marker appear inside markdown code blocks but do nothing when clicked #50781

Solution

• Added a new runnable config to LanguageConfig that disables runnables if set to false
• Changed config.toml config for Markdown to disable runnables

Testing

• Added tests to check that the runnables configuration is enforced
• Added tets to check that the default runnable configuration (true) is set, meaning, languages by default do load runnables to not change current behavior
• This changes can be checked by opening a demo file like the one on the linked issue, and opening a language file (.go, .rs) with tests to check that runnables are still being displayed on the editor.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Showcase

Left is markdown with language config runnable = false and right is latest zed nightly release.

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Release Notes:

• Added runnable option to disable runnables via language configuration.
• Fixed runnables showing up on markdown code fences

[AlvaroParker]

I wonder if having the ability to run markdown code blocks via the play gutter is indeed in the roadmap.

Having a README.md (or XYZ.md) file which typically show the steps to get certain things done via the corresponding code blocks. ie:

Install dependencies: bash npm ci

Run the service: bash npm run start

... and being able to click the play gutter on each of the blocks would be a life changer.

This doesn't address the comment at #50987 (comment)

This PR will disable runners in markdown entierly, which makes sense now since zed doesn't support runnables on Markdown files.

If runnables in markdown is in the roadmap, then a new approach for this bug is probably best. Maybe a similar configuration that avoids nested runnables (which is what's causing go runnables to appear on .md code fences).