Problem: Some injections (like markdown) allow specifying arbitrary
language names for code blocks, which may be lead to errors when
looking for a corresponding parser in runtime path.
Solution: Validate that the language name only contains alphanumeric
characters and `_` (e.g., for `c_sharp`) and error otherwise.
When trying to load a language parser, escape the value of
the language.
With language injection, the language might be picked up from the
buffer. If this value is erroneous it can cause `nvim_get_runtime_file`
to hard error.
E.g., the markdown expression `~~~{` will extract '{' as a language and
then try to get the parser using `parser/{*` as the pattern.
The official developer documentation in in :h dev-lua-doc specifies to
use "--@" for special/magic tokens. However, this format is not
consistent with EmmyLua notation (used by some Lua language servers) nor
with the C version of the magic docstring tokens which use three comment
characters.
Further, the code base is currently split between usage of "--@",
"---@", and "--- @". In an effort to remain consistent, change all Lua
magic tokens to use "---@" and update the developer documentation
accordingly.
