[Linter::Haml] Avoid haml-lint STDERR warnings

[NickLaMuro]

Injects a "stub" 'parser/current' file (that should be appended to the Ruby $LOADPATH), which avoids the annoying loading the original, which includes the unskippable warnings like this:

warning: parser/current is loading parser/ruby27, which recognizes
warning: 2.7.2-compliant syntax, but you are running 2.7.1.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

in STDERR, which are flagged by the bot as errors (when they aren't useful to the end user).

This stub will just define the Parser::CurrentRuby constant manually using the conventions of the parser gem.

Example in the wild

ManageIQ/manageiq-ui-classic#7650 (comment)

(copied in case that goes away)

Checked commit
NickLaMuro/manageiq-ui-classic@386138b
with ruby 2.6.3, rubocop 0.82.0, haml-lint 0.35.0, and yamllint 3 files
checked, 1 offense detected

**

• 💣 💥 🔥 🚒 - Linter/Haml - Linter::Haml STDERR:

warning: parser/current is loading parser/ruby26, which recognizes
warning: 2.6.6-compliant syntax, but you are running 2.6.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

[miq-bot]

Checked commit NickLaMuro@186680b with ruby 2.6.3, rubocop 0.82.0, haml-lint 0.35.0, and yamllint
3 files checked, 2 offenses detected

lib/linter/haml.rb

• ❗ - Line 14, Col 26 - Rails/FilePath - Please use Rails.root.join('path/to') instead.

vendor/stubs/parser/current.rb

• ❗ - Line 21, Col 17 - Style/RedundantSelf - Redundant self detected.