Choosing an interpreter for running executors

Klio lifecycle commands need scripting language engine for running executors (builders, deployers, etc). Currently they are using *goya*, but it was chosen as a temporary solution, without evaluating other options.

Good interpreter should meet the following requirements:

- It has to be small - chosen engine is going to be included in every binary, it shouldn't add 3MB to each one of them.
- It should have good documentation for both itself and a language.
- It should have good standard library, otherwise we need to create it by our own.

### Which engines are taken into consideration?

Stats as at 16 Sep 2021.

| Package | Language | Stars | Watchers | Forks | Last Commit |
| ----------------------------------------------------- | ----------- | ----- | -------- | ----- | ----------- |
| [otto](https://github.com/robertkrimen/otto) | JavaScript | 6222 | 189 | 521 | 14 Jun 2021 |
| [goja](https://github.com/dop251/goja) | JavaScript | 2386 | 66 | 201 | 12 Sep 2021 |
| [go-ducktape](https://github.com/olebedev/go-duktape) | JavaScript | 778 | 28 | 90 | 26 Mar 2021 |
| [gopher-lua](https://github.com/yuin/gopher-lua) | Lua | 4289 | 149 | 477 | 29 May 2021 |
| [go-lua](https://github.com/Shopify/go-lua) | Lua | 2152 | 312 | 156 | 2 Mar 2021 |
| [golua](https://github.com/aarzilli/golua) | Lua | 555 | 34 | 193 | 7 May 2021 |
| [go-python](https://github.com/sbinet/go-python) | Python | 1303 | 43 | 128 | 14 Apr 2021 |
| [gpython](https://github.com/go-python/gpython) | Python | 517 | 21 | 58 | 18 Nov 2019 |
| [yaegi](https://github.com/traefik/yaegi) | Golang | 3816 | 50 | 183 | 15 Sep 2021 |
| [tengo](https://github.com/d5/tengo) | Tengo | 2422 | 57 | 141 | 5 Sep 2021 |
| [starlark-go](https://github.com/google/starlark-go) | Starlark | 1367 | 46 | 122 | 1 Sep 2021 |
| [anko](https://github.com/mattn/anko) | Anko Script | 1167 | 47 | 111 | 21 May 2020 |
| [go-php](https://github.com/deuill/go-php) | PHP | 42 | 811 | 92 | 1 Oct 2018 |

### Additional size of commands

Following numbers are based on interpreters integrated with *build* command, compared with
interpreter-less version.

| Package | Compressed | Uncompressed |
| -------------- | ---------- | ------------ |
| **goja** | 3.19MB | 7.63MB |
| **gopher-lua** | 1.13MB | 2.47MB |
| **otto** | 1.21MB | 2.81MB |
| **tengo** | 0.36MB | 0.86MB |

### Standard libraries

Relevant features supported by standard libraries:

| Package | File System | Exec | HTTP | Regex | JSON | YAML | Print | Date & Time |
| -------------- | ----------- | ---- | ------ | ----- | ---- | ---- | ----- | ----------- |
| **goja** | no | no | no | yes | yes | no | yes\* | yes |
| **gopher-lua** | yes | no | no | no | no | yes | yes | yes |
| **otto** | no | no | no | yes | yes | no | yes | yes |
| **tengo** | yes | yes | no\*\* | yes | yes | no | yes | yes |

\* Provided by an additional library\
\*\* Maintainers are open for idea of adding it

## Remarks

### goja

- Simplest to integrate.
- Too heavy - it adds 3MB to compressed binary.
- Has partial support for ECMA 2015+, but it's poorly documented which features are actually supported.
- JavaScript is a divisive language.

### gopher-lua

- Using it wasn't a pleasant experience. For example, using userdata as a table froze command without any errors.
- Awful API.
- IO API is too barebone to be useful.
- Lua has some quirks, it tend to be hard to use for new developers.

### otto

- Substantially harder to work with than goja.
- No support for ES2015 or later.
- Errors tend to be a little bit cryptic.
- JavaScript is a divisive language.

### tengo

- Uses custom language, but it's quite nice and easy.
- Documentation could be better, but isn't the worst either.
- Has some issues with converting Golang types to Tengo, but they can be circumvented by
marshalling/unmarshalling.
- Pretty good standard library.
- It's really small, it adds only 0.36MB to compressed binary.

### gpython

- There is no documentation how to integrate it into golang app.
- Seems to be no longer actively maintained.
- Passing data into VM is not supported.
- Seems to be no way to extract data from a VM.
- Everybody loves python.

### yaegi

- Full-blown Golang interpreter, not suitable for scripting.

No source file or test is named. Review the size and standard-library tables and the remarks for goja, gopher-lua, otto, tengo, gpython, and yaegi; compare them with the stated requirements, then document a decision and rationale for the interpreter to use in place of goya.

Written by the indexing model from the issue text.