Rework TemplateEngine to support code

[IvanIvanoff]

Changes

Rework the TemplateEngine run/{1,2} and run_generate_positional_params/{1,2} functions to also support code templates with {% %}.

As part of the rework:

• Split the functionality into multiple modules;
• Add better documentation and tests;
• Rework the return format of the functions for better error handling;

Example:

template = "My name is {{name}} and 5 + 2 = {% 5 + 2 %}}"
Sanbase.TemplateEngine.run(template, params: %{name: "Ivan"})
"My name is Ivan and 5 + 2 = 7"

To provide non-empty environment for the code segments, provide the :env option:

env = Sanbase.SanLang.Environment.new()

env =
  Sanbase.SanLang.Environment.put_env_bindings(env, %{"a" => 1, "b" => "some string value"})

template = """
1 + @a = {% 1 + @a %}
The value of @b is {% @b %}
"""

Sanbase.TemplateEngine.run!(template, env: env) 
"""
1 + @a = 2
The value of @b is some string value
"""

Ticket

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have tried to find clearer solution before commenting hard-to-understand parts of code
• I have added tests that prove my fix is effective or that my feature works