[librustdoc] Reform lang string token splitting

Only split doctest lang strings on `,`, ` `, and `\t`. Additionally, to
preserve backwards compatibility with pandoc-style langstrings, strip a
surrounding `{}`, and remove leading `.`s from each token.

Prior to this change, doctest lang strings were split on all
non-alphanumeric characters except `-` or `_`, which limited future
extensions to doctest lang string tokens, for example using `=` for
key-value tokens.

This is a breaking change, although it is not expected to be disruptive,
because lang strings using separators other than `,` and ` ` are not
very common

compiler/rustc_error_codes/src/error_codes/E0761.md

@@ -2,7 +2,7 @@ Multiple candidate files were found for an out-of-line module.
 
 Erroneous code example:
 
-```ignore (multiple source files required for compile_fail)
+```ignore (Multiple source files are required for compile_fail.)
 // file: ambiguous_module/mod.rs
 
 fn foo() {}