chore(conversion): 'to_float' and 'parse_float' should share implementation

[pront]

Summary

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

If these two diverge, it opens up the path to subtle bugs for users. The to_float method is more generic and could eventually replace parse_float but there's no harm keeping both for now as long as they don't diverge in behavior.

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.
• For new VRL functions, please also create a sibling PR in Vector to document the new function.

References

Partially addresses: #1120

[jszwedko]

Nice clean-up!