fix: escape backslashes in values and keys so they round-trip

[greymoth-jp]

ini.parse(ini.stringify(x)) is not stable when a value or key contains a backslash.

unsafe treats \ as an escape character: in the unquoted branch it unescapes \\, \; and \#. safe only escaped ; and #, never the backslash itself, so a backslash that is written verbatim is read back as an escape sequence. Two backslashes collapse to one, and a backslash placed before ; or # gets swallowed.

const ini = require('ini')

// a value of two backslashes comes back as one
ini.parse(ini.stringify({ k: '\\\\' }))      // { k: '\\' }
ini.parse(ini.stringify({ p: 'C:\\\\tmp' })) // { p: 'C:\\tmp' }

This makes safe escape the backslash as well, so it is the inverse of unsafe. Section names are left unchanged: they use \. to escape literal dots in key paths, so safe is called with escapeBackslash = false for the section to avoid doubling those structural backslashes. The existing snapshots are byte-identical.

Round-trip tests are added in test/bar.js (two backslashes, a Windows-style path, \;/\#, and a backslash in a key). npm test passes at 100% coverage, and reverting the one-line change makes the new cases fail.

This is the behaviour #9 had in mind: "a \\ needs to be a \ and not an escape".