fix: escape backslashes in set_key so values round-trip

[Noethix55555]

Summary

set_key writes single-quoted values but only escapes single quotes, not backslashes. Because the single-quoted-value parser treats \ as an escape character (decoding \\ -> \ and \' -> '), any value containing a backslash is corrupted on round-trip through get_key.

import dotenv

dotenv.set_key(".env", "PATH", r"C:\Users")
dotenv.get_key(".env", "PATH")   # before: "C:Users"  (backslash lost)

dotenv.set_key(".env", "RE", r"\d+")
dotenv.get_key(".env", "RE")      # before: "d+"

This affects Windows paths and regular expressions in particular.

Fix

Escape backslashes before escaping single quotes in set_key:

value_out = "'{}'".format(
    value_to_set.replace("\\", "\\\\").replace("'", "\\'")
)

This mirrors how the parser unescapes values, so writes round-trip.

Tests

Added a parametrized round-trip regression test (test_set_key_roundtrip_with_backslashes) covering C:\Users, \d+, \, a\b, a\\b, and a backslash+quote combination. Verified it fails on main and passes with the fix. The existing test_set_key cases and the rest of the suite remain green.

Closes #661