Add locale for es-MX

[ChemaBautista]

Basic information

• What is the language code according to ISO 639-1?
• What is the country code according to ISO 3166-2?

(please replace LANG and COUNTRY in the issue title accordingly)

• Provide a list of all holidays, i.e. days "which are defined by law on which business or work are suspended or reduced"
• Are holidays moved to a different date, e.g. if they fall on a saturday/sunday/...? If yes, which holidays and what is the algorithm?
• Are holidays observed on a different date (is there a substitute holiday), e.g. if they fall on a saturday/sunday/...? If yes, which holidays and what is the algorithm?
• If possible, provide the legal sources which define the holidays.

For each holiday

• What is the official name?
• Is it a regional or nation-wide holiday?
• If regional, what are the regions the holiday is observed in? (Note: currently only regions defined in ISO_3166-2 are supported)
• Is it a fixed or variable date?
• If fixed, provide the date in format MM-DD
• If a holiday has a variable date: What is the algorithm to calculate it (e.g. nth <weekday>/<day> in <month>,...? If it depends on the easter date: Is it the western or orthodox easter date?
• If the holiday is proclaimed by the government: Can you provide the source?