Update `syntax/README.md` and replace `\<`, `\>` in syntax files

[niten94]

Some of the changes done in this pull request are written below:

1. Replacing \< and \> with \b in syntax files - The nanorc files where most syntax files originated use \< and \> to match word boundaries, but they were still contained in few files. \> in objc.yaml and perl.yaml are retained, since I can't determine the purpose of the patterns.
2. Replacing list in runtime/syntax/README.md to syntax files with hardcoded colors - It was mentioned in #3606 (direct link needed since account deleted?) that the list doesn't match. I'm not familiar with the file types where hardcoded colors are used, so I modified the list only.
   • I mentioned asciidoc.yaml but there's only a rule which changes font style, so it's excluded.
3. Merging sage.yaml with python3.yaml - I'm not exactly sure what Sage is but syntax file differences only seemed to be old Python features, so they are merged.
   • Scripts seemed similar when I looked at these links:
     • Do I have to rewrite all my code in Python instead of Sage?
     • Standalone Python/Sage Scripts
     • The Pre-Parser: Differences between Sage and Python