The expressions were parsed from right to left, and it was incorrect and
cumbersome. Now they are processed from left to right, with a simpler
and correct handling of precedence rules.

The vm function call syntax has been changed to set the function
before the input arguments on the stack, as to follow the declaring
order in languages.

Also improve the setup of golangci-lint.

* fix (parser): don't panic if assign of define untyped value

In case of defining or assigning to untyped value, the type has
to be taken from the source value instead of the target value.

The vm test coverage has also been slightly improved.

* fix and simplify Token.Name()

* improve parser errors

* chore: add linters and some lint fixes

Configure some golangci-lint linters to get the code quality right.
Apply the first fixes.

Next step will be to add github actions to run lint and tests in
github CI.

* chore: more lint, fixed comments and variable names. no semantic change.

* chore: add Makefile

This makefile is intended to be used as a local substitute to github
actions.

The `import` statement is now parsed. It only provides minimal
support for the `fmt` package (only `Println` symbol is defined).

This should be sufficient to pass a few tests.

Full support of package namespaces, source and binary imports will be
supported later, based on this work.