fix(parser): support bash-style single-quote splices in values

[simoncraf]

Summary

This PR adds support for bash-style single-quote splices inside .env values.

Before this change, python-dotenv failed to parse values like:

VAR='I'"'"'m a student'

Even though this is valid Bash and evaluates to:

I'm a student

After this change, python-dotenv accepts that form and parses it correctly.

Closes #544

What changed

• Added support for the '"'"' splice pattern while parsing single-quoted values
• Kept existing \' handling unchanged
• Added parser-level coverage for the new form
• Added dotenv_values coverage for the same behaviour

Example

This now works:

a='b'"'"'c'

and is parsed as:

b'c

Notes

This change is intentionally narrow in scope.

It does not try to implement general shell-style concatenation. It only supports the specific splice form commonly used in Bash to embed a literal single quote inside an otherwise single-quoted string.