fix(tree): set width 100% on tree item so custom renderer can do layout

[dtanp-rft]

Description

[part=label] in ef-tree-item not set width to 100% so when using custom renderer, we can't layup element easily e.g. using flex.

For example, https://codepen.io/dtanp-rft/pen/BawpzRv
It's not possible for space-between to work if width is not set to 100%.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings