Enable `Find Usages` menu for custom file type different from `TEXT` and `textmate`

[angelozerr]

Today Find Usages menu is enable:

• when FileType is TEXT or textmate
• when the editor / PsiFile return a non empty UsageTarget https://github.com/JetBrains/intellij-community/blob/7eac72e7741d7a13fe3221972a191847f68d30de/platform/usageView/src/com/intellij/usages/UsageTargetUtil.java#L39

The first condition is managed with LSP4IJ, the second by the language support himself (ex : JAVA).

This restriction with TEXT or textmate has been done to avoid overriding the Find Usages of the language (ex: Java). This behavior is annoying when a custom FileType provide support for LSP but don't contribute to Find Usages with usageTargetProvider extension point.