feat(formatter): group similar name at-rules

[idoros]

This PR changes the formatter behavior in order to group same name at-rules while maintaining spacing for other at-rule types.

Before this PR, the following code would remain unchanged:

@namespace "abc"
@st-import "./a.st.css"
@st-import "./b.st.css"
@property --a;
@property --b;

After this PR, the formatted code will be:

@namespace "abc"

@st-import "./a.st.css"
@st-import "./b.st.css"

@property --a;
@property --b;