[skip changelog] Allow library names to start with number (arduino#1136)

Previously, the Arduino library specification required the library.properties `name` field value to start with a letter.
Before Arduino IDE 1.8.4, library folder names starting with a number were not supported. Because the library folder for
Library Manager installations is named according to the library.properties name value, it was necessary to apply the
same restriction to the name value. It was determined that enough time has passed since the Arduino IDE 1.8.4 release to
reflect this change in the Arduino library specification.

Author: per1234

docs/library-specification.md

@@ -40,8 +40,8 @@ The library.properties file is a key=value properties list. Every field in this
 otherwise below, **all fields are required**. The available fields are:
 
 - **name** - the name of the library. Library names must contain only basic letters (A-Z or a-z) and numbers (0-9),
-  spaces ( ), underscores (\_), dots (.) and dashes (-). They cannot start with a number. Note that libraries with a
-  `name` value starting with `Arduino` will no longer be allowed
+  spaces ( ), underscores (\_), dots (.) and dashes (-). They must start with a letter or number. They must contain at
+  least one letter. Note that libraries with a `name` value starting with `Arduino` will no longer be allowed
   [addition to the Library Manager index](https://github.com/arduino/Arduino/wiki/Library-Manager-FAQ) as these names
   are now reserved for official Arduino libraries.
 - **version** - version of the library. Version should be [semver](http://semver.org/) compliant. 1.2.0 is correct; 1.2