feat: Add globalName entrypoint option.

[jviney]

Overview

Implements #2004

The IIFE name is not currently configurable for content scripts and unlisted scripts. The variable name can be important when inserting scripts with world=MAIN or directly via a <script> tag to ensure there is no conflict with an existing variable name on the page.

This PR is a followup to the discussion in #1897 where we decided to add a globalName option.

Questions:

• The globalName property is available on all entrypoints - should we restrict this to just content scripts and unlisted scripts in the type definitions?
• Is there any benefit to having the globalName option available for other entrypoint types?
• How shall we indicate that the default value will switch to the behaviour of false in a future WXT version? Would this version change the generated output for other script types too (eg: background, popup), or will we leave them generating a named IIFE.

Manual Testing

Specify the globalName option on a content script or unlisted script entrypoint and check the generated JS output matches what's expected for the given option.

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[aklinker1]

• The globalName property is available on all entrypoints - should we restrict this to just content scripts and unlisted scripts in the type definitions?

Only content scripts and unlisted scripts are bundled as IIFEs, so this option is only relevant for them.

• Is there any benefit to having the globalName option available for other entrypoint types?

No, for the above reason.

• How shall we indicate that the default value will switch to the behaviour of false in a future WXT version? Would this version change the generated output for other script types too (eg: background, popup), or will we leave them generating a named IIFE.

You don't need to indicate anything in this PR. For the PR that makes the breaking change, add a section to the PR explaining the change and migration path for people.

https://github.com/wxt-dev/wxt/blob/main/CONTRIBUTING.md#breaking-changes-policy

[jviney]

Ready for another review.