Rework remote task synchronization (#18746)

Reworks the way tasks are stored, accessed and synchronized in the
`project`.
Now both collab and ssh remote projects use the same TaskStorage kind to
get the task context from the remote host, and worktree task templates
are synchronized along with other worktree settings.

Release Notes:

- Adds ssh support to tasks, improves collab-remote projects' tasks sync

crates/task/src/lib.rs

@@ -140,8 +140,13 @@ impl FromStr for VariableName {
         let without_prefix = s.strip_prefix(ZED_VARIABLE_NAME_PREFIX).ok_or(())?;
         let value = match without_prefix {
             "FILE" => Self::File,
+            "FILENAME" => Self::Filename,
+            "RELATIVE_FILE" => Self::RelativeFile,
+            "DIRNAME" => Self::Dirname,
+            "STEM" => Self::Stem,
             "WORKTREE_ROOT" => Self::WorktreeRoot,
             "SYMBOL" => Self::Symbol,
+            "RUNNABLE_SYMBOL" => Self::RunnableSymbol,
             "SELECTED_TEXT" => Self::SelectedText,
             "ROW" => Self::Row,
             "COLUMN" => Self::Column,