feat(custom-esbuild): expose current target to index html transform

[mattlewis92]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

The current target is not exposed to the index html transform

What is the new behavior?

The current target is exposed to the index html transform to match the behaviour in the webpack builder which does expose the current build target

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information