$enable-shadows default value conflicts with the one in Bootstrap

[kikostadinov]

Describe the bug
Importing Bootstrap after Kendo-Bootstrap, as advised in Kendo documentation, will set $enable-shadows to true and apply predefined decorative box-shadow styles on Bootstrap components, while $enable-shadows is set to false in Bootstrap by default.

To reproduce
Steps to reproduce the behavior:

1. Create a React project with a Bootstrap input
   <input type="text" class="form-control" placeholder="Username" aria-label="Username" aria-describedby="basic-addon1">
2. Import Kendo-Bootstrap and then Bootstrap
3. Inspect the input. There will be the following declaration:
   box-shadow: inset 0 1px 2px rgba(0,0,0,.075);

Expected behavior
The box-shadow should not be present by default.

Affected package (please remove the unneeded items)

• theme-bootstrap

Affected suites (please remove the unneeded items)

• Kendo UI for jQuery
• Kendo UI for Angular
• Kendo UI for React
• Kendo UI for Vue
• Telerik UI for Blazor

Affected browsers (please remove the unneeded items)

• All

Build system information (please remove the unneeded items)

• Not Applicable

Additional context
DEMO. Uncomment the the following code to see the expected behaviour:

// $enable-shadows: false;

[joneff]

Fixed with #4201 -- we now use a prefixed variable that can be linked to bootstrap's should customers want to