Fix #1250. Support Ctrl+click to open an include file

The following commands are recognized for including a file
- include, input, subfile
- (sub)?(import|includefrom|inputfrom)

src/providers/definition.ts

@@ -4,6 +4,7 @@ import * as path from 'path'
 
 import {Extension} from '../main'
 import {tokenizer} from './tokenizer'
+import * as utils from '../utils'
 
 export class DefinitionProvider implements vscode.DefinitionProvider {
     extension: Extension
@@ -12,6 +13,33 @@ export class DefinitionProvider implements vscode.DefinitionProvider {
         this.extension = extension
     }
 
+    private onAFilename(document: vscode.TextDocument, position: vscode.Position, token: string) : string|null {
+        const line = document.lineAt(position.line).text
+        const escapedToken = utils.escapeRegExp(token)
+        const regexInput = new RegExp(`\\\\(?:include|input|subfile)\\{${escapedToken}\\}`)
+        const regexImport = new RegExp(`\\\\(?:sub)?(?:import|includefrom|inputfrom)\\*?\\{([^\\}]*)\\}\\{${escapedToken}\\}`)
+
+        if (! vscode.window.activeTextEditor) {
+            return null
+        }
+
+        let dirs: string[] = []
+        if (line.match(regexInput)) {
+            dirs = [path.dirname(vscode.window.activeTextEditor.document.fileName), this.extension.manager.rootDir]
+        }
+
+        const result = line.match(regexImport)
+        if (result) {
+            dirs = [path.resolve(path.dirname(vscode.window.activeTextEditor.document.fileName), result[1])]
+        }
+
+        if (dirs.length > 0) {
+            return utils.resolveFile(dirs, token, '.tex')
+        }
+        return null
+    }
+
+
     public provideDefinition(document: vscode.TextDocument, position: vscode.Position, _token: vscode.CancellationToken) :
         Thenable<vscode.Location> {
         return new Promise((resolve, _reject) => {
@@ -63,6 +91,13 @@ export class DefinitionProvider implements vscode.DefinitionProvider {
                     return
                 }
             }
+
+            const filename = this.onAFilename(document, position, token)
+            if (filename) {
+                resolve(new vscode.Location(
+                        vscode.Uri.file(filename), new vscode.Position(0, 0)
+                    ))
+            }
             resolve()
         })
     }