feat: add git context item category

What does this MR do?

feat: add git context item category

• adds support for local_git context items
• supports rendering for diff and commit gitType items
• improves rendering for new/unknown components (stop using default values/icons etc, instead hide elements if we don't have a match)

These local_git context items will allow users to select a commit, or their current changes, or a diff of their current branch against main, and attach it to chat in one go.

This MR is followed by feat(GlDuoChat): view context item content/details (!4648 - merged)

Screenshots or screen recordings

before	after

(don't default to icon / type for unknown items, instead hide these elements so we get minimal but correct info)

Integration merge requests

Support for this new UI will be added in VSCode here: gitlab-vscode-extension!1964 (merged)

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.