fix parse interpolation error

[heychenfq]

hello, when i use postcss-less as less preprocessor parser in my project, i get some parse error.
here is my example:
@{prefix}-title{\ncolor: red;};

i try debug internal code, i found that when parse interpolation, token match would not stop util not match @|{|}, in my example, the final match token is @{prefix}-title{，i guess this works unexpected，so i try to fix it. make the { token just match once, in my project, it works fine.

Please check one:

• New tests created for this change

This PR:

• Adds new API
• Extends existing API, backwards-compatible
• Introduces a breaking change
• Fixes a bug

[shellscape]