Add template variable ${configDir} for substitution of config files directory path

[sheetalkamat]

• ${configDir} on any paths is replced with config files directory instead of making it relative to file its declared in.
• This is allowed on all the isFilePath options, paths, files, include and exclude
• Not allowed to pass from command line

7b1a848 is just tests change
9fcd0d6 actual change

Fixes #57485

TODO:
Actual string to use for replacement from design meeting:

• configLocation
• configFolder
• configDirectory
• projectLocation
• projectFolder
• projectDirectory

[weswigham]

Does a --showConfig on a config with paths like these show the input ${configDir} or the resolved disk paths?

[weswigham]

...Do we need to allow escaping $ so you can actually use a $ in a path, rather than as one of these templates? Like if I (somehow - the character is forbidden on NTFS) had an actual directory named ${configDir} - how do you escape from the pattern back to a literal interpretation?

[sheetalkamat]

Does a --showConfig on a config with paths like these show the input ${configDir} or the resolved disk paths?

https://github.com/microsoft/TypeScript/pull/58042/files#diff-dfa255ce269b3c80a416b7c133db82a20292076b8c53370637b1aeaff7b8e7f3R110 shows resolved path and i think its useful to show it like that.

[sheetalkamat]

...Do we need to allow escaping $ so you can actually use a $ in a path, rather than as one of these templates? Like if I (somehow - the character is forbidden on NTFS) had an actual directory named ${configDir} - how do you escape from the pattern back to a literal interpretation?

Not sure. i think we should keep this simple as otherwise we need to ensure all the paths are normalized to contain escaped $ we can choose other template pattern as well its a TODO but we shouldnt have to do escaping

[jakebailey]

Long term, I do wonder if we'll need some other flag here given we'll introduce more substitutions, but thankfully this is internal!