Feature/autocomplete improvements for Headers

[fantpmas]

Description

This PR

• adds case-insensitive autocomplete for Headers (e.g. accept will now match Accept)
• makes autocomplete trigger within words (e.g. language will now match Accept-Language)
• fixes the bug where autocomplete for Header values was not triggered on folder level

Resolves #3776 #4355

Contribution Checklist:

• [x] The pull request only addresses one issue or adds one feature.
• [x] The pull request does not introduce any breaking changes
• [x] I have added screenshots or gifs to help explain the change if applicable.
• [x] I have read the contribution guidelines.
• [x] Create an issue and link to the pull request.

Screenshots

JIRA