fix(theme/#1933): Scope matching precedence

[bryphe]

Issue: Our scope-selection strategy was not correct in some cases - particularly, the precedence in which we would apply the scopes.

For example, in #1933 - with the dracula theme - there'd be a case where a token would have the following textmate scopes:

• support.function.console.js
• meta.function-call.js
• source.js

For the dracula theme, there is a source selector that is intended to be a fallback - however, it was taking precedence over some of the other theme selectors that should've been applied, like meta.function-call.

This would cause the entire source file to be highlighted with the source scope selector, instead of the more specific ones.

Defect: We apply selectors in reverse order (first, we apply any selectors matching source.js, and then meta.function-call.js source.js, etc...) - this gives us right precedence. However, if we had a match, we were failing to 'fall-back' to a more specific selector.

Fix: Add a test case to reproduce, and fall back to override with more specific scopes.

This looks to fix a couple of related issues:

• no syntax highlighting #1933 - no syntax highlighting with OneDark / Dracula for some files
• incorrect syntax highlighting for tsx files #2006 - incorrect syntax highlighting for tsx files
• HTML syntax highlighting is not displayed #1879 - html syntax highlighting not displayed
• Nested Syntax Support #1878 - nested syntax highlights not shown

Thanks @CrossR for the helpful tool improvements in #2255 and @thismat for the investigation to narrow down the issue in #1933 !

dracula theme - before:

dracula theme - after: