This improves the implementation in gh-106643.
Previously, venv passed "(<prompt>) " to the activation scripts, but we want
to provide the original value so that users can inspect it in the
$VIRTUAL_ENV_PROMPT env var.
Note: Lib/venv/scripts/common/Activate.ps1 surrounded the prompt value with
parens a second time, so no change was necessary in that file.
The activation scripts generated by venv were inconsistent in how they changed the shell's prompt. Some used `__VENV_PROMPT__` exclusively, some used `__VENV_PROMPT__` if it was set even though by default `__VENV_PROMPT__` is always set and the fallback matched the default, and one ignored `__VENV_PROMPT__` and used `__VENV_NAME__` instead (and even used a differing format to the default prompt). This change now has all activation scripts use `__VENV_PROMPT__` only and relies on the fact that venv sets that value by default.
The color of the customization is also now set in fish to the blue from the Python logo for as hex color support is built into that shell (much like PowerShell where the built-in green color is used).
Jim Porter