allow parsing more legal environment variable names

[svix-jbrown]

This loosens the key parser to allow all POSIX-legal and reasonable names (basically, ignoring the things that don't make sense in a text file like BEL, and ignoring leading digits as recommended by POSIX).

Note

Some more background here for anyone reading who doesn't know; there's a lot of nonsense on the Internet, like this very-popular Stack Overflow thread that conflates environment variables with shell variables and other concepts. Environment variables are very loosey-goosy -- they're just an array of strings, which by convention should contain at least one =! POSIX 2024-8.1 specifies that an environment variable name may be any character other than NUL or =. This is actually pretty common (for example, most Windows environment variable names have % in them, and many enterprise-y Unix applications use environment variables containing the : character). Shells (like bash) have a notion of shell variables, which are more restrictive than environment variables; for example, bash's are limited to [a-zA-Z_][a-zA-Z0-9_]*. Many shells provide a way to convert a shell variable to an environment variable, like bash's export word. However, even in a shell you can set arbitrary environment variables by using the syntax env 0scary=whatever ./my-program.

This PR is a superset of what's described in #158.

Unfortunately, this does somewhat conflict with #138, in that some of the bourne shell substitution metacharacters are also legal environment variable name characters. I personally would get more value out of supporting all legal environment variable names than I would out of supporting bash-style ${foo:-default_if_unset} substitutions, but I can see both arguments.