Closes#30.
Pattern: identifier optional_spaces open_parenthesis
The implementation uses `"("` instead `(syntax open-parenthesis)` as
`open-parenthesis` is defined from `(open-paren (or "{" "[" "("))`. As a result,
the regular expression would also highlight array and dictionary variables.
The regular expression does highlight signal definitions with parameters,
though, as they follow the same pattern of function calls. This could be avoided
by ignoring expressions started with "signal" (or by adding a custom rule to
highlight signal definitions).
Doom Emacs seems to flatten the package's file structure when installing from
GitHub. The new gdscript--get-package-file-content-as-string function will try
to load the file without the relative directory component if it can't find it
in, e.g., a data/ subdirectory.
