Allow relocating Aegisub's files with environment variables

[CoffeeFlux]

Adds three environment variables, read once at startup:

Variable	Effect
AEGISUB_DATA_DIR	Replaces ?data, the bundled files (automation includes and autoload, etc.)
AEGISUB_USER_DIR	Used for all per-user files (settings, user scripts, logs, autosaves, caches) instead of the platform's default locations
AEGISUB_CONFIG	The config file to use instead of the one in the user dir

Empty values count as unset, and relative paths are resolved against the working directory at startup.

Why

• Running an uninstalled build. On Linux ?data is compiled in as the install prefix. Since the Lua support modules (moonscript.lua etc.) are now only loaded from ?data, an uninstalled build can't run automation scripts. AEGISUB_DATA_DIR=<source tree> fixes that, and the CLI end-to-end tests in Headless CLI #670 need it.
• Clean or separate profiles. AEGISUB_USER_DIR gives a throwaway or per-project profile without touching the real one, the way BLENDER_USER_RESOURCES, KICAD_CONFIG_HOME or Chrome's --user-data-dir do. Headless CLI #670 will add matching command-line flags for CLI mode, using the same code.

Details

One place for the per-user directories. AEGISUB_USER_DIR and Windows portable mode both go through the new agi::Path::SetUserDir, which currently points ?user and ?local at the given directory. Caches move along with everything else; mpv's --config-dir is a known trap for not doing that. Any per-user directories added later then only need adding there. For example, #264's ?state needs to be, or AEGISUB_USER_DIR and portable mode would leave logs and autosaves in the default location.

Precedence: AEGISUB_USER_DIR, then Windows portable mode (a config.json next to the executable), then the platform default. AEGISUB_CONFIG applies whichever user dir wins, so it can be combined with a clean user dir.

?dictionary isn't moved by AEGISUB_DATA_DIR, even on platforms where it's derived from ?data.

Directories that are files. If AEGISUB_DATA_DIR or AEGISUB_USER_DIR points at a file, it's ignored with a warning in the log. Otherwise agi::Path would silently use the file's parent directory.

Testing

Tested on macOS with the GUI:

• With all three set, the log and hotkey.json were created in the given user dir, the config was read from the given file, and an autoload script in the given data dir's automation/autoload was loaded. Nothing new appeared in the real profile.
• With AEGISUB_USER_DIR pointing at a file, the warning was logged and the default location used.

The Windows-specific bits (_wgetenv and the portable-mode precedence) are only compile-tested by CI.

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