Assist to extract out variables assignment in if/match

[Veykril]

Given code like the following:

let mut completion = ...;
if let Some(snippet_cap) = ctx.snippet_cap() {
    completion = completion.insert_snippet(snippet_cap, pat);
} else {
    completion = completion.insert_text(pat);
}

the assist should turn it into:

let mut completion = ...;
completion = if let Some(snippet_cap) = ctx.snippet_cap() {
    completion.insert_snippet(snippet_cap, pat)
} else {
    completion.insert_text(pat)
};

cc #6964 (comment)

[Veykril]

Fixed in #7130