Add a tool that lists identical contexts in a syntax file

Preventing duplication is not necessarily preferable, but it may be useful for simplifying future modifications.

```sh
utils/detect-identical-context.py data/syntax/*.xml | sort
```

```
data/syntax/4dos.xml: ['cmdsimpleNoChecks', 'cmdTakeAFileName', 'cmdFilesystemOperation']
data/syntax/4dos.xml: ['conditionFileTest', 'conditionRight']
data/syntax/4dos.xml: ['fixedDo', 'countedDoBy']
data/syntax/asciidoc.xml: ['passthrough delimited', 'verbatim delimited']
data/syntax/bash.xml: ['BuiltinGetoptsOpt', 'BuiltinLetArg']
data/syntax/bash.xml: ['FindPathThenPopMaybeGlobAny', 'ExprDblBracketValueMaybeGlobAny']
data/syntax/brightscript.xml: ['function_ret', 'sub_ret']
data/syntax/cabal.xml: ['license', 'licenseFile', 'maintainer', 'author', 'name', 'description', 'copyright', 'homepage', 'bugReports']
data/syntax/csv-whitespace.xml: ['Column0Spaces', 'Column1Spaces', 'Column2Spaces', 'Column3Spaces', 'Column4Spaces', 'Column5Spaces', 'Column6Spaces', 'Co...