Default 'roots' field of language config

Previously roots needed to be specified by every language and `[]` was
used as an explicit default. Root files don't make sense for every
language (for example TOML) so I think we should allow languages to
not explicitly mention the key and have the `[]` default automatically.

helix-core/src/syntax.rs

@@ -101,6 +101,7 @@ pub struct LanguageConfiguration {
     pub file_types: Vec<FileType>, // filename extension or ends_with? <Gemfile, rb, etc>
     #[serde(default)]
     pub shebangs: Vec<String>, // interpreter(s) associated with language
+    #[serde(default)]
     pub roots: Vec<String>,        // these indicate project roots <.git, Cargo.toml>
     pub comment_token: Option<String>,
     pub text_width: Option<usize>,