feat(interpreters): add an existing Python virtual environment as an interpreter (#2076)

Refs #1907

Add Python Interpreter → Existing environment: Browse picks the environment folder, which is resolved right away (pyvenv.cfg, bin/python or Scripts\python.exe) and added to the interpreter list as "Python X.Y (<env name>)", then remembered. Picking the folder rather than the executable keeps the environment's identity, since the native file picker resolves the venv's python symlink to the base interpreter. Works for `python -m venv` and uv environments; a broken environment shows an error instead of being added. Probing runs off the command thread.

Co-authored-by: Inf1mum <[email protected]>

src/platform/locales/text/en.json

@@ -1,4 +1,7 @@
 {
+  "Select Python Environment Folder": "Select Python Environment Folder",
+  "Checking interpreter": "Checking interpreter",
+  "Cannot run this Python interpreter. Select a working Python executable or virtualenv folder.": "Cannot run this Python interpreter. Select a working Python executable or virtualenv folder.",
   "Native server declined rename": "Native server declined rename",
   "document is too large or is not a regular file": "document is too large or is not a regular file",
   "document is too large": "document is too large",

src/platform/locales/text/zh.json

@@ -1,4 +1,7 @@
 {
+  "Select Python Environment Folder": "选择 Python 环境文件夹",
+  "Checking interpreter": "正在检查解释器",
+  "Cannot run this Python interpreter. Select a working Python executable or virtualenv folder.": "无法运行此 Python 解释器。请选择可用的 Python 可执行文件或虚拟环境文件夹。",
   "Native server declined rename": "语言服务器拒绝重命名",
   "document is too large or is not a regular file": "文档过大或不是普通文件",
   "document is too large": "文档过大",