Closes #12628

Adds a `$ZED_LANGUAGE` task variable that resolves to the language name
of the active buffer (e.g., "Rust", "Python", "Shell Script"). This lets
tasks adapt behavior based on language without parsing file extensions.

Use cases from the issue:
- Pass syntax highlighting language to external tools (e.g., `rg --type
$ZED_LANGUAGE`)
- Adjust comment wrapping width per language
- Handle extensionless files and untitled buffers that have a language
assigned

VS Code provides equivalent functionality via
`${command:activeEditorLanguageId}`. Neovim exposes it as `&filetype`.

## Changes

- Added `Language` variant to `VariableName` in
`crates/task/src/task.rs`
- Populated from `buffer.language().name()` in
`BasicContextProvider::build_context`
(`crates/project/src/task_inventory.rs`), following the same pattern as
`File` and `Stem`
- Updated test fixtures in `crates/tasks_ui/src/tasks_ui.rs` to include
the new variable
- Added `ZED_LANGUAGE` to the variable list in `docs/src/tasks.md`

## Testing

Updated the existing `test_task_variables` test in `tasks_ui` to verify
`ZED_LANGUAGE` resolves to "Rust" and "TypeScript" for the respective
test buffers.

This contribution was developed with AI assistance (Claude Code).

Release Notes:

- Added `$ZED_LANGUAGE` task variable that exposes the current buffer's
language name

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>