fix: support comments in multiline input

[domodwyer]

Support parsing lines that contain comments & quotes.

Previously this input:

# Start of .env file
# Comment line with single ' quote
# Comment line with double " quote
 # Comment line with double " quote and starts with a space
TESTKEY=test_val # A '" comment
# End of .env file

Would fail the regression test (test_issue_12):

running 1 test
test test_issue_12 ... FAILED

failures:

---- test_issue_12 stdout ----
thread 'test_issue_12' panicked at 'should succeed: LineParse("# Comment line with single ' quote\n# Comment line with double \" quote\n # Comment line with double \" quote and starts with a space\nTESTKEY=test_val # A '\" comment\n# End of .env file\n", 181)', dotenv/tests/test-multiline-comment.rs:21:14
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/panicking.rs:142:14
   2: core::result::unwrap_failed
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/result.rs:1805:5
   3: core::result::Result<T,E>::expect
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/result.rs:1055:23
   4: test_multiline_comment::test_issue_12
             at /Users/dom/Documents/rust/dotenvy/dotenv/tests/test-multiline-comment.rs:21:5
   5: test_multiline_comment::test_issue_12::{{closure}}
             at /Users/dom/Documents/rust/dotenvy/dotenv/tests/test-multiline-comment.rs:8:1
   6: core::ops::function::FnOnce::call_once
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

After this commit, the env var is set as expected (test_val).

Fixes #12.

---

• fix: support comments in multiline input (0bcc818)

  Fixes the line parser to respect comments when evaluating quote state.
  
  Fixes allan2/dotenvy#12.
  

[LeoniePhiline]

Here, a regression is introduced:
This line makes KEY='ABC#DEF' get truncated to KEY='ABC.

The # can be escaped for the line parser as \#, but the value parser does not handle escape characters within strong quotes, resulting in ABC\#DEF rather than ABC#DEF as final value.