`env`: remove `rust-ini` in favor of a dedicated `.env` parser

[tertsdiepraam]

We currently use rust-ini to parse .env files for env's -f/--file flag. We could move to a dedicated .env parser. There are some libraries for this available on crates.io, but we'll have to evaluate them.

[tertsdiepraam]

Most dotenv parsers seem to have the following features:

• Values can be quoted with some escape sequences
• # marks a comment unless it's quoted or escaped
• Some have some variable expansion
• They might have multiline values.

As far as I can tell, there's not really a spec. Everyone just goes with "it's bash-like".

Examples:

• python-dotenv
• Ruby's dotenv
• dotenv-rs (seems a bit unmaintained, there are some forks available)

[tmccombs]

FWIW, Systemd has this description for how EnvironmentFile is interpreted:

The text file should contain newline-separated variable assignments. Empty lines, lines without an "=" separator, or
           lines starting with ";" or "#" will be ignored, which may be used for commenting. The file must be UTF-8 encoded. Valid characters are unicode scalar values[7] other than noncharacters[8], U+0000
           NUL, and U+FEFF byte order mark[9]. Control codes other than NUL are allowed.

           In the file, an unquoted value after the "=" is parsed with the same backslash-escape rules as unquoted text[10] in a POSIX shell, but unlike in a shell, interior whitespace is preserved and
           quotes after the first non-whitespace character are preserved. Leading and trailing whitespace (space, tab, carriage return) is discarded, but interior whitespace within the line is preserved
           verbatim. A line ending with a backslash will be continued to the following one, with the newline itself discarded. A backslash "\" followed by any character other than newline will preserve the
           following character, so that "\\" will become the value "\".

           In the file, a "'"-quoted value after the "=" can span multiple lines and contain any character verbatim other than single quote, like single-quoted text[11] in a POSIX shell. No backslash-escape
           sequences are recognized. Leading and trailing whitespace outside of the single quotes is discarded.

           In the file, a """-quoted value after the "=" can span multiple lines, and the same escape sequences are recognized as in double-quoted text[12] of a POSIX shell. Backslash ("\") followed by any
           of ""\`$" will preserve that character. A backslash followed by newline is a line continuation, and the newline itself is discarded. A backslash followed by any other character is ignored; both
           the backslash and the following character are preserved verbatim. Leading and trailing whitespace outside of the double quotes is discarded