feat: add pre-defined envd template

[kemingy]

• fix feat: create an pre-defined dev env in one command #1685

Try it: envd new -t uv

[Copilot]

PR Overview

This PR adds a new command "new" to create a build.envd file from pre-defined templates, addressing issue #1685. It also introduces a dedicated template directory and updates file utilities and bootstrap stages to support this feature.

• Adds pkg/app/new.go with the new command implementation.
• Introduces DefaultTemplateDir and TemplateFile in pkg/util/fileutil/file.go.
• Updates bootstrap and app command registration to include the new command.

Reviewed Changes

File	Description
pkg/app/new.go	Implements new command that creates build.envd using pre-defined templates.
pkg/util/fileutil/file.go	Adds DefaultTemplateDir and a TemplateFile function to manage template files.
pkg/app/bootstrap.go	Adds a bootstrap stage for adding pre-defined templates.
pkg/app/app.go	Registers the new command within the app commands.

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

pkg/util/fileutil/file.go:142

• Duplicate definition of TemplateFile detected; if a TemplateFile function already exists in this file, remove the duplicate to avoid conflicts.

func TemplateFile(filename string) (string, error) {