Prepare for ESLint v9

[m5r]

Describe the issue
ESLint v9 comes with a new default configuration format that's incompatible with the previous one. The good thing is that ESLint v8 (the current version used in cht-conf) supports both formats and allows us to prepare the code for this migration.
Projects configurations need to prepare for this change too because cht-conf uses their ESLint configuration to lint their code when compiling app settings.

Describe the improvement you'd like
A progressive migration could look like:

• Migrate cht-conf's ESLint config
• Migrate ESLint config generated by cht-conf in initialise-project-layout command
• Warn users of the deprecation of .eslintrc if we notice this file in a project and point them to ESLint's migration guide
• Update ESLint to v9 along with the next cht-conf major (v5?)
• Crash cht-conf with a meaningful error message when user runs cht-conf v5 in a project with an old ESLint config

Describe alternatives you've considered
N/A