Escape backslashes so set_key values round-trip

[eeshsaxena]

Summary

Fixes #661.

set_key (and dotenv set) writes single-quoted values but only escapes single quotes, not backslashes. Because the single-quoted-value parser treats backslash as an escape character, values containing backslashes did not round-trip:

• an embedded \ collapsed to \ on read;
• a value ending in a backslash (e.g. a Windows path like C:\) wrote a trailing \', which the parser read as an escaped quote - swallowing the closing quote and corrupting the whole file (subsequent lines became unparseable).

Reproduction (before)

import dotenv
dotenv.set_key(".env", "A", r"a\b")   # embedded double backslash
dotenv.get_key(".env", "A")            # -> "a\b"   (one backslash lost)

dotenv.set_key(".env", "B", "C:\\")    # trailing backslash
dotenv.get_key(".env", "B")            # -> None    (line unparseable)

Fix

1. In set_key, escape backslashes before single quotes (order matters) so the written value round-trips.
2. Update the _single_quoted_value tokenizer from (?:\'|[^'])* to (?:\[\s\S]|[^'\])* so every \<char> (including \) is consumed as one escaped unit. Without this, even a correctly-escaped trailing backslash ('C:\') merges its second backslash with the closing quote. The [\s\S] keeps the previous acceptance of newlines after a backslash inside single quotes.

The change is scoped to single-quoted values (the form set_key emits). Values without backslashes are unaffected, so all existing set_key output expectations are unchanged.

Tests

• test_set_key_backslash_round_trip - round-trips Windows paths, regexes, embedded/trailing backslashes, and backslash-before-quote.
• Two test_parse_stream cases - an escaped backslash decodes to one backslash, and a trailing escaped backslash does not consume the closing quote.

Full test_parser.py + test_main.py pass (124 passed). (test_cli.py::test_run_with_invalid_cmd fails on main too - a pre-existing Windows-only error-message assertion, unrelated to this change.)