feat: update to use Infracost v0.10 (actions v2) (#65)

* docs: change examples to use hcl by default

* chore: use v0.10.0-beta.1 release

* docs: fix version in setup README

* test: fix example tests

test: fix tests to checkout the correct version for the setup task

test: don't compare against the base branch in tests

test: update tests so they also update the var files to test changes

test: fix example test golden files

test: fix private terraform module test

* docs: make examples consistent with GitLab (#64)

* docs: make examples consistent with GitLab

* docs: update example step comments

* docs: update golden files and examples to be more consistent

* docs: add commented out example for TFC

* docs: use consistent terminology with migration guide

* docs: update examples to use `--flag=value` instead of `--flag value`

This gives better error messages if the value is an empty env variable.

* docs: simplify comments

Co-authored-by: Ali <[email protected]>

* docs: add setup comment for Docker image

* docs: move setup action first

* docs: fix indentation

* chore: use v0.10 version

Co-authored-by: Hugo <[email protected]>
Co-authored-by: Alistair Scott <[email protected]>