workspace: Add configurable window title formatting

[jknlsn]

Summary

• Add window_title_format and window_title_separator settings for customizing the window title
• Expose ${projectName}, ${fileName}, ${filePath}, ${relativePath}, ${fileStem}, ${remoteName}, ${remoteHost}, ${appName}, and ${separator} as the native Zed window-title variables
• Keep the collab ↗/↙ indicator hard-appended
• Translate VS Code window.title / window.titleSeparator aliases into Zed's canonical variables during settings import, including ${appName} passthrough support and mapping VS Code's ${activeEditorMedium} onto Zed's ${relativePath}
• Gate path-, remote-, and release-channel-derived lookups on whether the active template references the corresponding variable, so the default template avoids the extra work and still uses the existing last_window_title cache to skip redundant platform updates

An alternative approach is at #49638. I went with ${variable} names over $ZED_*, canonical Zed variable names with VS Code aliases handled at import time, kept ${separator} as the only structural token, and left ${dirty} out of scope.

Implementation reasoning and notes

Variables considered but not included

• collab/shared indicator variable: kept hard-appended instead. Folding it into the template would need whitespace/separator logic the other variables don't need, and I'd rather not bake those semantics in without a concrete need for it.
• dirty variable: The glyph choice and interaction with OS-level edited indicators (like macOS's titlebar dot via set_window_edited) felt like a separate design call I wasn't confident making here.

A separator-style config for each of collab/shared/dirty would solve the above, but I didn't want to add too many additional settings initially.

Performance

update_window_title is on a hot path, so two short-circuits apply:

• Path-, remote-, and release-channel-derived lookups (absolute_path, relative-path string, file_stem, remote_connection_options, ReleaseChannel::try_global) are gated on template.contains(...) via WindowTitleNeeds::from_template, so the default template avoids the extra work.
• The existing last_window_title cache still skips the platform call when the rendered title is unchanged.

Screenshots

Settings Editor entries for window_title_format and window_title_separator:

Customized Zed window titles in AltTab app switcher (remote, local, empty):

Self-Review Checklist:

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

Closes #14534

Release Notes:

• Added configurable window title formatting with window_title_format / window_title_separator, including ${appName} / ${remoteName} / ${remoteHost} variables and VS Code window.title import support