fix: support embedded union interfaces

feat: add Position.Add()

Useful for sub-parsers.

Allow custom types to be used for Pos, EndPos and Token.

Specifically, these fields can now be of any type that is convertible
from the internal Participle types.

fix: union elements that were pointers would panic

Fixes #255

Simplify error handling.

Expose participle.ParseError and lexer.Error, remove some redundant
functions.

Genericise the parser.

`Build(&Grammar{})` becomes `Build[Grammar]()` and
`err = parser.Parse(filename, &grammar)` becomes
`grammar, err := parser.Parse(filename)`.

Genericise the parser.

`Build(&Grammar{})` becomes `Build[Grammar]()` and
`err = parser.Parse(filename, &grammar)` becomes
`grammar, err := parser.Parse(filename)`.

document parameter "filename" for the Parser methods

document parameter "filename" for the Parser methods

document parameter "filename" for the Parser methods